I would avoid the tankless propane heater for your RV. This is mainly because you do not get steady water pressure from your pump, and that your inlet water temp can be as low as 45F in northern campgrounds.
While a tankless heater can easily 'warm' the 70F water from a Texas, southern AZ or California campground, where the ground stays at 45 - 55F all year long, the heater will not warm water from 45 to 105F at a reasonable rate in gallons per minute.
A 10 gallon tank heater can provide warmer water for a long time, at a rate of 3 GPM if you like. The 10,000 BTU input from the gas will re-heat at the rate of 10 gallons per hour, or 1 gallon every 6 minutes. The electric heat option will also heat at about 2 gallons per hour (90F or from 45F to 155F). You would mix 1 gallon of really hot water with 1-2 gallons of campground not heated water to get your supply to the shower at 105F. The tank is held at 140F, so that less hot water is required to take a shower, until the tank gets cold.
Good luck,
Fred.
