I don't think you really want to access either the black or grey tanks unless absolutely necessary. We had to do it in our 2001 Adventurer and it was a royal PITA.
First off you have to remove a couple of the side panels. They're held in place by screws on a top and bottom flange. To get to the screws you have to remove the trim belt.
Once the trim and panels are removed you have to remove a set of sheet metal panels between the main floor and the sub floor. Once the screws are removed you have to get a thin putty knife under the sheet metal and slowly break the caulking type seal. The sheet metal panels are thin and the glue/caulking is strong. It takes lots of patience to keep from destroying the panels.
Once you gain access to the tanks you'll find there's very little room to work on them. You have to be a bit of a contortionist just to get to the plumbing connections.
We had a leak where the fitting from the toilet meets the black tank. We didn't know it until we had driven about 250 miles and were pulling in the drive. As we went up the slope you could see black gummy crud dripping out the back of the motorhome.
It took several hours to access the tanks, and another couple hours to flush out the smelly goo. Once everything was clean It was a real chore to get into the area and find the problem.
Once we found the problem we quickly realized the repair was beyond our capabilities. We didn't have the tools or components to replace the broken connection to the tank. In the end we took the motorhome to the Forest City service center. Even though we had already gained access to the tanks it took the better part of a day to repair the connection. We stayed overnight and they leak tested and reinstalled the panels the following day.
It's not a process I would wish to repeat. If we have a similar problem in the future the motorhome will go directly to Forest City.
Hikerdogs
2013 Winnebago Adventurer