Grey wolf 26dbh holding tanks

Can't really help you totally with your model, but "generally" the valves are located at the tank, and that may help you with their position. The grey tanks sometimes have the valve at the end of a pipe run, but the black will almost always be as close to the tank as possible to keep the s**t in the tank instead of collecting in a pipe. As to the fresh tank, the fill hose also "generally" is kept short with as few curves as possible. If you can see where it goes into the floor you can get an idea where it is in relation to the axles.

It may be possible to get a "print" of the tanks, floor, and frame from the mfg. I know that some mfgs don't do that, but I've found that most will for an owner.