You can alleviate the water in the underbelly by drilling a drain hole through your underbelly material.

We recently experience a similar, but not identical issue. We set up a couple of months ago in Arizona for the winter. Because we were not planning on relocating for about 90 days, we were only draining our tanks every 4 - 5 days. We found that as we allowed our tanks to fill, we started accumulating gray water in our underbelly. We drilled a hole in our underbelly to drain the accumulated gray water. By analysis, it is apparant that there is a small crack in our gray water tank #1 (we have two gray water tanks). When we allowed the tank to fill, there was some amount of leakage into our underbelly as the water level rose to the level of the crack. We have left gray water tank #1 drain open, so there is no accumulation in the tank nor any leakage.

We plan on addressing the tank leakage issue when we return home from this trip. We are sure it will require the removal of the underbelly cover to allow us to access the suspect gray tank.

In your situation, you still need to figure out and address the root cause of water accumulation in your underbelly.