I have the furnace out. It was easy to do. I took the base for the settee out to give me easy access to the water lines to the shower that parallel the delivery lines from the pump/water heater past the T's going to the bathroom sink and toilet.
I ran the pump to pressure up the system and water poured out between the frame rails under the RV. Both my wife and I checked for the leak with a video scope and by touch and can not identify the point where the water is leaking.
There are 2 valves in a cubby hole on the left side of the hallway. The valves appear to operate 3 ways. To drain the system, stop water from going on to the galley sink and to allow water to the sink. With the galley sink isolated, water still comes out under the body roughly where the supply lines lay in the foam core floor. I can't see any evidence of of water along the path of either line.
Today I plan to carefully run the RV up on some solid wooden ramps I made to gain some access to the underbody. Perhaps there are some lines hidden underneath. Anything is possible on a 17 year old RV. We found underwear and a sock under the shower.
If my efforts are still fruitless I will give Winnebago a call on Monday. Good suggestion.
I wanted a project and this neglected old girl is proving to be precisely that.