I had a similar problem with my WH chassis. In the back where the 50 amp cords comes in there is a panel of fuses that is hidden by a metal panel. Once I found these fuses there was a 40 amp fuse that was blown. This fuse has blown a couple of times since, I keep a couple of fuses in the coach. It seems you have checked all the battery connections so I'll bet you have a fuse panel you have not found yet.
"Life is a Journey. Enjoy the Ride"
2004 Winnebago Journey 39K w/ 330 Cummins on a Workhorse Chasis (rare but no problems yet).