On my trailer I had a similar leak. The cause ended up being that two of the corners on the black holding tank flange, which the valve screws to, had broken off. The other two screws were holding, and being that my valve arangement is wedged between the tanks, everything held together even when full, just got a small leak where the black valve mated with it's tank.
The broken corners were still there, hanging on the mounting screws. I took everything apart and was able to glue the broken corners back on with a solvent glue rated for both PVC and ABS, since I wasn't exactly sure which I was dealing with. At this same time I replaced both dump valves since they had leaking interior seals.
As for access, a removeable panel was already on the bottom of the tank enclosure, though a previous owner had made some cuts to bend metal out of the way and enlarge the opening. I cut this flap of metal loose and had lots of room to work, and then spliced things together into one jumbo access panel when done. I did that about six years ago and its still working well.