First, there are many folks disjointed with a tankless heater in their RV. The isue is the low volume desired so the waste tank doesn't fill. At low volumes, the tankless units drop in and out of heating mode, makes for a bad shower.
Most electric tankless heaters, if not all, operate at 240V. Residential tankless are usually orificed for natural gas but I'm sure propane models are available.
My suggestion would be to buy a 6 Gal. RV heater off E Bay and, if you need to, you can find them that run off either propane or 120V electric.