I just took a quick look at Rheem's website and didn't see a 20 gallon lowboy, looks like smallest I saw was 30 gallon 240 volt dual 4500 watt elements. AO Smith makes a 20 gallon available with 2500 watt 120 volt element model #EJCS-20, it's only about 25" tall with all water connections on the side. AO Smith supposedly has an advanced glass tank lining, don't know if it's any better or worse than Rheem. It's rated for an 11 gallon/hour recovery based on having to raise water temperature by 90 degrees. The recovery rate on any water heater, gas or electric, tank or tankless is based on incoming water temperature which varies by different parts of the country.
I've seen this done in residential applications but not in an RV; pipe the two water heaters in series. The lowboy will have adjustable thermostats wheras the RV type has a non-adjustable preset to 130 degrees. Make the lowboy the workhorse and preheated reservoir for the RV water heater, set the thermostats to 100 degrees or so, leave the onboard water heater on gas only. When you turn the hot water tap on, you are only asking the gas model to raise the temperature by 30 degrees or so which should be easy. The effort is to make the gas model basically an on demand system and only asking it to raise the water temperature a few degrees. I'm guessing about the temperature settings, you would just have to experiment.
A 2.5KW 120 volt element WILL require a 30 amp single pole breaker and #10 wire. If you leave the electric element off in the onboard water heater, you're only adding a little over 8 amps to total load and shouldn't create a power management issue.
You will absolutely need to install one of the strap systems now required in many areas to prevent tipping over during travel. The T&P valve will need to be piped to the outside, in your case necessitate drilling a hole through the floor. You will need an electrical disconnecting means near the water heater, you can get one of the little pull-out models pretty cheap. If you ever have to winterize, it will need to be drained, these (actually all) tank models are made so they will never drain completely. They theorize that leaving a little water in the bottom helps keep some of the minerals in suspension and not cling to the tank. Another consideration is weight; the empty water heater is 68 lbs, add another 166 for the water, and by the time you add the piping and hardware, you're adding about 250 pounds directly to the pin weight. You'll have to determine what if any effect that will have.
