It won't heat and says, 21 and then A1. If the gas is empty, do I need to fill my main propane tank outside?

It may be the Thermocouple safety device. Here is some video help.

https://www.google.com/search?q=Attwood+propane+water+heater+won%27t+stay+lit

RV water heaters are mounted against an outside wall. Go outside and look for a removable cover that has a mesh vent installed.
Open this cover and the water heater should be inside.

If the water heater is propane your propane tank may be low of fuel. Also very cold weather may not let the propane produce adequate vapor to fuel the water heater. A low level of propane can make this problem worse.

If the water heater is electric, you may have too large a wattage draw from other appliances, when the water heater is trying to heat. Turn off items such as hair blowers, AC, and Microwave when the water heater is on. If a generator is providing the electricity it may not have enough output. If shore power is providing the electricity you need at least 30 AMPs.

Immediate danger!
You have a gas leak.
Propane is heavier than air and will settle on the floor.
The floor can be covered and ignite and you will not smell it while standing or sitting or laying in bed.
Absolutely open the door and leave trailer. Turn off the gas to the heater if the shut-off is outside.
Possibly you have a bad gas control valve. This should be repaired professionally.

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

Hi Bob,

You can determine if the tank is the problem by connecting a garden hose to the drain that is provided on most water heaters. Before doing this, be sure to shut of the electrical supply to the heater - otherwise you can damage heating elements if an electric type or waste fuel by heating an empty tank if a gas / propane fired type.

If you get a slow flow out of this drain, it means that somewhere in the cold water supply side to / in the tank - there is a problem. Inside the tank, the cold water is delivered by a "dip tube" that distributes the cold water to the bottom in the tank. Dip tubes are used in both electric and gas / propane fueled heaters. Here's a cut away view of a typical tank:





If the tube is clogged, or there is build up, scale or debris in the supply (cold water) side of the heater, flow rates will suffer. Likewise, if there is a problem with the output of the tank, it will directly affect flow as well.

If you get a good flow rate from the drain, then the issue is going to be on the hot water output side of the heater. Again, the same build up, scale, debris issues on the hot water out put pipe will cause problems with flow. Typcial tanks do not have dip tubes for the hot water side of the tank - as taking hot water from the top of the tank is most desireable and is furthest away from the cold water that rushes in to make up the hot hot water going out.

There's nothing really in the tank itself that can cause issues with flow outside of the dip tube, and piping to and from the tank.

Good luck!

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 main water off to the water heater, it may be a valve on cold side coming in to shut off. Next turn power off to the heater and drain the tank the drain will be located near the bottom of the tank.
You will need a water hose to connect to the drain to run water outside. Open a faucet hot side to allow you to drain the tank.

Upon reinstall connect the tank up and fill tank and open up hot side on one faucet at a time to purge out air from system before turning power back on. Make sure you have water flowing from each faucet's hot side before turning power back on.

Rick, if the tank is normally plumbed for a horizontal position, and you have it standing vertically, then you are feeding liquid propane to your pilot light, hence the frosted hose etc., and a tendency for the pilot light to go out as it has too rich of a fuel to burn. There should be no vent to allow air in as propane is pressurized, and only attains zero pressure at 40 below zero farenheit. The valve you speak of should have been the liquid vent valve which is used during filling only, and is closed during any other use. Your supply valve should be in the wide open position. The pressure relief valve is only operational when the tank has been overfilled or overheated. You should have a pressure regulator between the tank and the appliance you are trying operate. Hope this helped. Marvin

error code 21 is malfunction of air intake filter switch. You need to check and make sure the air filter door is properly seated and that the front panel is properly installed

The tankless water heaters require larger gas supply lines because of the on-demand consumption. While I don't know your units rating, most Propane Tankless Waterheaters require 2" supply lines. Moreover, pressure is directly related to temperature which sounds like your problem entirely; consider the run of the gas pipe from the tank and the ambient temperature. Now you see why plumbing is a hard profession to fake... =)