[Carl n Susan]

The location of the tanks depends on your floor plan. The black and grey can be on either the DS or ODS. Look for the toilet down pipe in the passthru area to give you a clue. My 3700RL has the black tank on the ODS. Apparently the 3120/3121 series have the black on the DS.

You only have to drop a couple of feet of coroplast to get to the black and shower tanks. The valves you are looking for are near the rear of the dropped frame section. Alternatively, you can cut a flap with the opening to the rear to access the valves. You probably want to drop an edge of the coroplast to figure out where to cut. By the time you have done that, it is just as much work to just drop the back of the coroplast.