nosebreaker,
My memory isn't what it used to be, but when you were looking for an RV, didn't you say you had plans to sell it after your trip?
I'm thinking that if that's the case, you're looking to do a fair amount work which, in my opinion, might negatively impact resale value/curb appeal of the rig.
Now, if you're going to keep it long term, then that might be a different story. But if resale means anything to you, you might think twice about this mod. I know I wouldn't want an on-demand system in my motorhome. Heck, my wife wants me to remove the one I installed in the house! LOL Funny story, but everyone else in the family was able to adapt to it's required method of operation but not her. She always complains about uneven temps...too hot...too cold. Go figure. I offered to help her in the shower but she declined...but that's another story! LOL
Oh...I just thought of this, but it's a long shot. IF your rig has an outside shower, make sure the hot & cold valves are OFF. If the valves are open a bit, the hot water can mix with the cold at the outdoor shower handle even if the valve on the handle is off. Perhaps this might be shortening your "hot water" availability. Just a thought.
Well, whatever you decide, please keep us updated as to your experiences. There are lots of us interested to hear of your results. :)
Good Luck,
~Rick