If it's like somewhat older trailers, the aluminum skin covers a thick hunk of plywood. You would remove two nut/bolt/washer assemblies from a piece of angle iron running across the frame, drop the angle iron, and slide the plywood forward out from under the fresh water tank. The tank will have to be supported so you don't damage any of the wires and hoses. Make sure the tank is empty, it's pretyy heavy when full of water.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.