If you can't find a model number, I remember mine was on a paper tag that I could not read
The next best thing is measure it and compare to the specs of a new Atwood water heater
Most likely a 6 gallon
You may want a DSI with electric heat if you don't have already I believe it's the same size and not much more money
There is always the new tankless water heaters for rv s now which are much more in cost
I do a lot of dry camping and found that a tankless was not the best route for me
I kept with my 6 gal DSI, changed my thermostats so I'm heating the water to 140 degrees then added an automatic thermostatic mixing valve to the back of the water heater which mixes in cold water and I get 9 gallons of hot water out of a 6 gallon water heater