How do I check my hot water heater

Could be your igniter or possibly your control board. The red light not being on indicates you most likely have an electrical issue. Check your breaker as well. If you nee any parts this site will have them all

http://www.suburbanrvparts.com/

check the gas pipe carefully spiders like to make home in there and clog it

Sounds like you have a hydro-generator on this unit. Check the output on the hydro-generator it should be about 1.5V DC. If not clean the inlet screen and check your water pressure. Water pressure less than 30 PSI will cause issues. If you have 1.2V-1.5V DC make sure the power switch is on. If the power switch is on replace the control unit. I sell parts for Bosch units only!

Why is it always Rheem problems?
Every water heater problem is Rheem.
And Rheem doesn't post any service manuals, like the other two big manufacturers, AO Smith-American, and Bradford White.

Copy following link for troubleshoot by gas control valve.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Scroll up and find pressure switch heading among links.

You can install gas control valve yourself.
http://waterheatertimer.org/pdf/How-to-replace-water-heater-gas-valve.pdf

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

Check for a bad fuse in panel first, then with a 12 volt test light or multimeter, check for power in to switch, as well as out when in on position if it's easy to get to. If not, turn switch on and test for voltage at brown wire to thermostat terminal, that comes from inside at back of water heater from switch inside. If it has cut-out switch in plastic sleeve attached, be sure to check for power on both ends of that cut-out switch. If power only in but not out, replace it. As I said,that brown wire comes from switch, so if no power to it, check at switch. The other brown wire goes back to connector on ignition board. If power is there, turn switch off, take an eraser on end of a pencil and rub contacts on ignition board/module to clean them off, as well as spray electrical cleaner on contacts of connector to make sure you have a good connection. Check all ground connections as well, and if you have 12 volts at brown wire to connector on ignition board but still no spark, replace ignition board/module.

Are you sure you don"t have hydro operation turned on?

First check thermal cut-off that is connected between incoming power wire and thermostat terminal. (located inside little clear plastic sleeve) Easiest way to do that is to remove it from the circuit temporarily. Just pull it off thermostat terminal, pull it apart from other end, and just connect female spade from incoming power wire directly to thermostat terminal. Then see if water heater ignites. If it does, and all works well, get new thermal cut-off. That cut-off is a safety feature in case of flame up at burner, it will shut off gas supply. It is not reset-able, just works once then needs to be replaced.
If that doesn't cure it, take a 12v test light, or a multimeter, and test for power at gas valve, when water heater is trying to ignite, or at ECO terminal where red wire runs from left ECO terminal to gas valve. If power is getting to gas valve, and you don't hear or feel it click, make sure ground connection is sufficient as well. If you have 10.5-13.5 volts to gas valve, and it's not opening and ground is good, then it's faulty and should be replaced. If it is clicking, (opening) but no gas coming through, you may have a plugged orifice on gas valve, provided you have cleaned burner tube of any spider webs or debris previously. If no voltage from red wire, start back tracking with meter or test light to other terminal on ECO, then to terminals on thermostat as well. You'll see how feed wire (short jumper) goes from thermostat, over to ECO. Remember, you will have to hold test light or meter probe at connection and have someone turn on switch to get a reading, which will only appear when it's trying to ignite. Let me know what you find.

Most electronic ignition models only show the red light if there is a problem, (did not ignite) for one reason or another. To start with, make sure your propane is on, and you have a good flame at stove top burner, just to be sure the water heater is getting propane, and all air is bled from the line, which may cause water heater to go into check. (red light) If it does, shut switch off, and retry. Let me know what happens. As far as pressure relief opening and emptying the water heater, and your sure it's not a faulty pressure relief valve, then there's 2 things to start with checking. 1st, does the water seem extremely hot, thus a possible faulty thermostat &/or ECO . Secondly, if the incoming water pressure is too high, that will also cause it to leak bad. Try installing a pressure regulator (usually 40 psi.) at your hose connection. Let me know what you find.

Make sure propane is on and getting to your stove burners ok, first, then try switching it back on. If it fails to ignite, shut switch off and try a couple of times, if still no ignition, get back to me.

sounds like you are getting an ignition failure code. Check the inside of the unit for evidence of a leaking HEX.