If the tank you have found is white, it is a FWT. If you have a strong magnet and can push up the plastic easily, work towards the front and back with plastic pushe'd up using the magnet. There are steel crossmembers up there that the tanks sit between. Once you have found the crossmembers cut your inspection hole to see where to drill. (Not the tank side of the crossmembers
. ). When I was looking inside my underbelly I stuffed some high density foam around the tank so it would not slide around up there. Also while I was in there I secured wiring, fixed the poor connections to the slide motor. Tightened some clamps and foamed in the holes where everything passed thru the floor. Hoses wiring etc. It was defiantly worth it for me to drop the plastic and see what was in there.