I fill the tank up, but it seems too light to be full, so I drain it and it only has about 1 gallon if that in it. Whats wrong?

Some hot water means upper element is good, reset button is pushed in and not tripped, overheating is not issue, wires connected at top of tank are probably good, and unit getting power, but not necessarily full power. Otherwise we have not ruled out anything else.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#new

Check for crossover.
http://waterheatertimer.org/Crossover.html

Check for leaks: turn off all taps and put ear against pipe.

Gene
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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

follow this steps and fix it. God bless you
Drain a tank:

1. Shut off power at the electric panel and post a sign warning others not to turn it on.
2. Close the cold-water supply valve and open a hot-water faucet somewhere in the house (to speed draining).
3. Attach a garden hose to the drain valve and run it outside the house.
4. Open the drain valve and allow all water to drain out.
5. To refill the tank, close the drain valve tightly and open the cold-water supply valve. Also open a nearby hot-water faucet. When a steady stream of water flows from that faucet, the tank is full and the faucet can be closed.
6. Once the tank is full, turn the electrical power back on.

Test and replace a relief valve:

1. Lift the spring lever on the valve to fill a small cup. Check the cup for sediment.
2. If no water spurts out, or if water continues to drip after the valve is released, replace the valve.
3. Cut power at the electric panel and post a sign warning others not to turn it on. Close the cold-water supply valve.
4. Drain a few gallons of water from the tank.
5. Unscrew and remove the discharge pipe if used.
6. Loosen the relief valve with a pipe wrench, then remove the valve by hand.
7. Apply pipe tape to the threads of the replacement valve and screw it into the tank by hand. Tighten it with a pipe wrench. Screw the discharge pipe (if any) into the valve outlet.
8. Refill the water heater and restore electrical power.

just turn off the gas valve for pilot light, turn off cold water to water tank. drain system. when refilling open cold water to fill tank, wait at least a 1/2 hour to 45 minutes then relight water heater. you should be fine!

82V52-2 is typical 240Volt 50-gallon electric water heater.
Water heater has to be full of water before turning on power.
This is true for gas and electric water heater.

Once water pipes are connected, you are ready to fill tank.
Turn on cold water shut-off so water enters tank.
Open bathtub spigot on hot side.
When tank is full, water comes out of tub spout full stream.
Let the water run until there is no more air coming out.
Next, open TP lever on water heater for a moment, and this will bleed out last pocket of air.
TP valve is located on upper part of tank, on the side or on top. It is a brass valve with silver-colored lever. Lift lever for a moment and water will exit tank down the overflow tube.
Water heater is full.
Turn on 240Volt circuit breaker going to water heater.

Other links with electric water heater information:
http://waterheatertimer.org/How-to-replace-water-heater-drain-valve.html
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#cleanelectric
http://waterheatertimer.org/How-it-works.html

DSi light coming on is to alert you that the water heater didn't light on propane for one reason or another. Sometimes people just forgot to turn propane on or are out of propane. What happens when you shut it off and turn it back on? (listening and watching from outside with cover open) DO you see or hear sparking?

As far as black water showing full when just emptied, it is most likely a piece of toilet paper inside the tank that is hanging up from top probe on inside of tank, and touching lower probe, thus giving a full reading. This is very common, at all levels. You can buy a rinse wand and rinse inside of tank through toilet to try and wash it off, or live with it for now, and add lots of water or cleaner deodorizer to tank , or buy probe cleaner, or even the old standby, some laundry soap added to tank , then fill tank close to full with water and rock trailer to swish it around, then drain it. Most times, waiting until next dump, and adding lots of water beforehand, will fix it.

Turn the gas valve to pilot and leave it there. This way the pilot will stay on but it wont try to heat and you dont have to re light the pilot. Turn off the water supply valve to the heater. Open the drain valve at the bottom of the heater and hold the lever open on the pop off valve towards the top of the tank. This will allow air into the tank and it will drain faster. If there is not a floor drain nearby you can hook a garden hose to the drain valve. When you are finished do not turn the gas valve back on until the heater is full of water. Open sink faucets when filling tank with water to avoid excess pressure.

Double check that washers inside fittings attached to back of tank, are not wedged or pinched in there restricting flow. Drain tank, relieve water pressure prior, obviously. The other thing is to check that the by-pass is allowing good flow in to tank when in normal operation mode. Have seen those only go half way inside valve, but lever is at wide open setting. If all that checks out, try removing the check valve on tank, and replace it with a 1/2" galvanized pipe nipple, ( or certified plastic/nylon fitting same size) and reconnect water line direct to that, without a check valve. That should do it if nothing else was wrong. Now you have a choice, either change it or leave it out if everything seems ok, but double check that hot water isn't siphoning back to another utility such as toilet if you leave it out and toilet is close. Feeling that cold line (inlet) when water is hot should tell you. I've seen the check valve in both locations, hot (at top of tank) and cold (at bottom of tank -inlet) Not 100% sure why, right from factory that way, but I say, put it on bottom fitting if required at all. Let me know how you make out.

'Rheem 81VP15S point of use' is a single-element, 15 gallon, 120V electric.
Other 'point-of-use' sizes range from 2-1/2 to 6 gallon to 20 gallon.

Electric water heaters have 2 parts: thermostat and element. One is not functioning correctly. Most likely the element caused by sediment build-up.

How to determine if tank has element or thermostat problem:
Do not turn off water going into tank. Put hose on drain valve. Open drain valve. Does water come out like open spigot? If water does not flow, then heater is likely clogged with sediment.

Solution 1: If water flows out like a spigot, the thermostat is probably bad. Read: How to replace thermostat.

Solution 2: Water does not flow, so element is surrounded by sediment build-up and cannot heat water. You need to clean out tank with shop vac, and put in new element ... plan full day and 3 trips to hardware store. Simply draining tank will not remove sediment. Sediment is chunky and will not come out voluntarily.

Buy element before starting:
-Turn off electricity
-Open cover on side of tank, remove insulation (these have to be put back for proper operation of thermostat)
-The are 2 electrical parts >>the element and the thermostat which is located above element
-Element wattage is printed on element and also shows on label on side of tank
-I think Rheem element is held in place with 4 hex bolts
-Take photo of element to hardware store so you have part on hand before removing old element
-If element is 1-1/2' diameter hex type, you need inexpensive element wrench from hardware store + the element

Drain tank and clean sediment:
-Read: To open drain on water heater so water comes out
-Read: How to clean sediment out of water heater

the hot water tank may be filled with scale buildup on the inside. drain a few gallons out of tank until the water runs clear. it is best to drain out water heater at least 2 times a year.

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.