Two 10 minute showers? Holy cow, I start pounding on the bathroom door of our regular house if I hear anyone the shower running for more than 5 minutes! :) I don't think I use that much water to wash my 36' motorhome!!! LOL
By comparison, our 10 gallon water heater in our motorhome easily supplies enough hot water for all 5 of us to take showers. Everyone has been "schooled" in the proper technique. :)
We have an on-demand water heater in our house. I think it works OK in that application, but I wouldn't want one in a motorhome. It's more wasteful of water and in some of our camping expeditions, I have plenty of fuel, but freshwater is sometimes more scarce.
~Rick