is it a 5th wheel?
on mine, the belly cover is held up by screws to the frame. they are put on is sections.
look closely and see if u can see a panel where there are screws across holding one panel to the other. this is an area u can take down and access the tanks. in other words, the panels may b rather long (front to back of the trailer). there will be some that are shorter. these will expose the tanks.
i have 2 gray tanks. i believe the fresh water is between the black and the rear gray.
my pump is in the cargo area drivers side near the stairs.