There is no access to the back of the Hydro-Hot in my coach which I suspect is typical. In order to remove the HH or do any maintenance in this area, it is necessary to cut an approximate 5" X 10" hole starting at the round drain hole underneath. Note that part of this hole is cut in the actual sheet metal of the HH. USE CAUTION NOT TO CUT THE PLUMBING, WIRING, ETC.
In my coach, the HH is supported front and back with a 2" angle iron bolted to structure in back and welded to another angle in front. Rather than drop the whole support assembly, I cut the weld on the back one, rotated it out of the way to cut the hole, fixed my leak, installed a cover plate over the hole and reattached the angle with bolts and a bracket. Now, God forbid, it will be easy to access this area for future repairs.
My leak was at the hot water outlet on the tempering valve. Both the hot and cold connections were finger tight. Held for 4 years without a problem.