Hi Lynn,
I'd look at a ten gallon that is electrical, propane, and (if you are in a self propelled RV) engine heat exchanger.
Tankless heaters are not yet ready for prime time in RV's.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.