Hi 1965GT and welcome to the Forums! It is exciting seeing how many people in Europe are leaning toward the vintage Airstreams. Does your screen name imply that you have a 1965 Globetrotter?
Your ask about doing an inspection or assessment inside the belly pan. I believe you'd want two other data points before you proceed to doing this. First, have as complete an inspection as possible of the plywood floor perimeter inside the trailer. Second, determine the condition of the visible parts of the frame. 20-, 30-, and 40-year old trailers existed under highly variable conditions. You will need to determine the source if you see any floor rot. Is the frame in good, fair, or really rough condition?
Belly wrap was very thin to begin with (0.025") and probably is corroded and fragile. Removing it will probably destroy it but take it off carefully to use the remnants for a pattern. Propane copper tubing penetrates through the belly wrap and routes along the outside bottom of the belly wrap. If an inspection is called for, I think you'd be happier cutting access panels in the belly wrap. Then you could rivet in place a patch of new aluminum sheet somewhat larger than the opening you made. Take a look at two tools I mention in this thread: http://www.airforums.com/forum...kin-26344.html