Hi Dock,
Gray tanks are usually accessible by taking down the coroplast underbelly. On a drop frame 5th wheel, the front section will usually come down without too much trouble. Harder to get it back in place unless you have help.
In the rear section, you may be able to take out a few screws from the off-door-side and peek inside. If that doesn't help, you will likely need to cut the coroplast. One approach is to cut 3 sides of a rectangle, leaving the front side intact. When putting back together, clean the surfaces thoroughly and use Gorilla Tape.