Bob,
The valves are not behind the UDC. The valves are on the output of each tank (1 black and 2 grays). The valves are cable-actuated. A cable connects to each valve and then back to the handle in the UDC.
So, your tanks are under the floor, between the frame rails. Please call Heartland Customer Service at 877-262-8032 to find out the location of the tanks.
To access the tanks, you will have to remove the corrugated plastic underbelly material in the area of each tank. Then remove some flex-foil insulation and fiberglass insulation.
If you are doing this repair on your own, once you establish the tank locations, I suggest you do an inspection of each tank valve before you tear into the repair. You may find out that it's not a leaky valve but something else. And maybe with just one tank, not 2 or 3.
Best of luck.
Jim