Is your lack of hot water just when using the shore electricity, or does it not heat on propane as well? My rig has the 10 gallon heater and for a time it was weak to non-existent hot water on electricity and I simply started using the propane until I got it fixed. Needed a new switch, but I changed out the electric heating element and the anode at the same time just to be sure it will go another decade. I can't imagine that your heater would not be easily repaired.
By the way, aren't the tankless water heaters all gas (propane) only??? If you do go that route, get one with induced draft like this:
http://www.adventurerv.net/girard-ta...r-p-27164.html
Chuck