Kelly the real issue is leaks. The belly is insulated with fiberglass insulation and once this insulation gets wet it holds the water and causes the rusting of the frame crossmembers and corrosion of the aluminium belly pan while in contact with the steel of the frame. If the floor is not rotted, left unchecked it will only be a matter of time before the wet insulation laying against the wood of the floor will rot out the floor. Most often the floor rots from below up. If the trailer does not have leaks to the inside the lower beltline trim is allowing water under it and into the belly. You can seal this trim and patch over the belly openings but this kind of repair is only a stop gap measure and to fully correct it the belly should be dropped, remove the wet insulation and replace with an alternative choice after repairing any frame rust. The frame can be painted with a rust preventative paint and replace the corroded belly pan with new.
Secondly once you have the floor up around the toilet you may be able to see how much damage there is to the frame but the black tank itself may obstruct any view you need. You can drill holes and install new rivets near the old corroded ones to hold the belly up there.
essentially it's the same as home repair unless you open it up fully you will never really know how bad it is. I knew I had some crossmember damage on mine but once I opened up the belly I discivered how bad the frame really was. It ended up being easier and better to pull the shell and build a whole new frame.
Here is some information for you to look at regarding products.
POR15, Inc. - Stop Rust Permanently - Repair Gas & Fuel Tanks
Insulation for Less : Prodex : Reflective Insulation : Pole Barn Insulation : Metal Building Insulation : Bubble Insulation
Inland RV Center Inc.
Inland RV Center Inc.