A rear reapit is alway the toughest to get to. Once the shell is complete at the factorey the builers start with the rear of the coach and work forward. This creates many issues when bathrooms need to be removed, or in your case interior walls need to be accessed.
The only real advice I can offer is to get the screwgun out and go slow. I just replaced the refer in my 78 MH and I had to take apart the cabinet it was installed in and much of the celing and overheads. It was all layered on each other and I had to get 1 lousy rivet out to make it all come apart for access.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato