Well, it was the tempering valve. Once my mechanic got it removed the hot inlet side was completely closed off except for a pin hole in the middle. The mix outlet was about 50% closed off as well. Nobody in our area had the valve in stock, they could all order it, and nobody had it for much less than AquaHot so I found one on Amazon with over night prime shipping for $78. Lowes does show they carry it for $8.75, but none of the stores within 100+ miles had it in stock.
We fired up the diesel heat and in about 15 minutes I had hot water coming out of the faucets! So I can finally take a hot shower in my camper now.
We're going to take it camping tonight and enjoy the hot water. Tomorrow it goes up to What Cheer, IA for the big flea market the following weekend. The wife will spend 5 days in it then and not have to fight the crowds at the shower house.
At this point I think we have pretty much everything of importance fixed, still need to get a in dash stereo installed, so will have to figure out the wiring diagram, but that's for another day.