Our coach is a 2004 and has an outdoor water bay where there is a freshwater manifold, a faucet, a freshwater tank fill hose, black/gray tank dump valve switches, etc. The access door for this water bay is the first basement door fore of the streetside rear tires. Does this sound similar to your RV?
If so, then the stainless steel plate that houses the manifold valves, etc. needs to have its screws removed so it can be tipped out for access to the dump valves behind the stainless plate and near the black/gray tanks. I have replaced both valves on our RV. Our two dump valves are electric - yours may be manual - with their switches located on the stainless steel plate... and it's a PITA to change these valves, I would add.