There are exceptions but there are basically 3 sizes of water heater common in RV's. 6 gallon, 10 gallon and psuedo 10 gallon (That is a 10 gallon heater in a six gallon hat) You can "Fake" the 10 psuedo 10 by putting a hotter thermostat (That's how they do it, they heat the water hotter, then mix in cold water to bring it back down to 120)
Lots of issues with that mixing valve though.
There are also a few different "Demand" type (instant) units. you can find more about them by searching threads... These burn Propane only for the most part but they give you "endless flow" at least till you know what overflows. (or the other tank runs dry).