IMPOSSIBLE to access from inside. The tanks are EXTERNAL to the trailer, being mounted underneath, with a large removable shroud covering them. The shroud is installed with sealer and screws and will be difficult to remove, and even more difficult to reinstall properly and get sealed up.
In case you haven't noticed, the Bigfoot
is like two boat hulls, one sitting normal and the other flipped over and joined together at the seam, It is all fiberglass underneath, with insulation and then a plywood floor.