Check these out... There is a typo in the chart....where it says rinse should say rise.
....example - the water coming into the water heater is 60 degrees F so a 45 degree rise would produce unlimited hot water at 105 degrees F. Still possible to take hot showers at lower temps.....just mix less cold water(or none) at the shower head.
If your motorhome has 50 amp service, it's possible to get 240vac for the water heater by wiring to L1 and L2 without a neutral.
Also this thread is helpful...
Tankless water heaters on sale today at woot!