Garp--What year and model trailer do you have?
The design of the interface between the bellypan and the shell changed in ~1969. 69 and later models, the skin will come up on the sides and just rivet onto the shell. This interface is hidden by a piece of trim that will have to come off first. Also, if the trailer is a 1969+, the bellypan will probably be in multiple sections. you might be able to do all the inspection you want without disconnecting the belly from the edges where it meets the shell, just disconnect on the underside, and let the side pieces hang. When you see what a mess is hiding under there, though, you will probably want to replace all the insulation, and if the frame is in need of repair, you will likely have to completely remove the bellypan anyway.