The SeaLevel sensors come in 12" segments, that you cut to size. I suspect that you have the same gray and black water tanks that I have in my 07 Diplomat. I have a 62 gal. gray and a 40 gal. black. You can get to them by opening the panel in front of your curb side rear wheel. It is held shut by two Roberts head screws and is hinged to open like the other bay doors. The road side requires taking out the plumbing bay face panel, which is not as hard as it seems, as all of the plumbing connections have disconnects behind the panel. Unscrew your sewer hose hole plate and look straight up with a flashlight to see behind the panel.
On the curb side, the tanks are beveled, so that they slope to the road side for better dumping. On the road side they are approx. 9.5" high. I decided to put my sensors on the curb side, as far toward the center (of the side of the tank) as I could reach. I felt like if they were installed on the road side end, that they would give a very different reading, depending on how level you were on any given day. The result is that I do not get any reading for the first 25%, but after that they are accurate.
Luke and Kathy & Bootsie kitty, Morro Bay,CA
2007 Diplomat 40' SFT
2013 Honda CR-V "More is the enemy of enough."