In my '75 Argosy24 rear bath, the layout is similar to what TG Twinkie described. The dump outlet is inconveniently located in a hole through the frame extension that supports the rear bumper, with the black valve located inside the bumper trunk, the gray valve outside a bit forward of the dump outlet, and the city water connection in between (very close to the dump outlet.) The shape of the bumper combined with the Thetford outlet prevent me from using handy stuff like the clear elbows that let you monitor the wastewater flow, etc. but I've managed.
The regulator and relief valve are inconveniently located in the "dead space" in the aft streetside corner, between the closet and the under-sink cabinet. A very small, skilled, flexible person might be able to replace them without dismantling the cabinetry, but I certainly wouldn't try.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill