Hmm first thing first is what is causing the rot. Has to be a water leak somewhere, and if you can see rusty frame rot your sub-floor must have severe water damage also.
I just redone the front area and the bedroom area/ cargo hold on our 93 Excella.
I did not remove the entire belly pan, only what I needed to do my repairs, on the cargo area I removed the four screws holding the rear bumper on, drilled out the four rivets (1/8'') that hold the plastic bay for the sewer hose and it cam out. I then changed to a 3/16'' drill bit and began drilling out the rivets that hold the belly pan up. I just kept drilling the rivets and letting the belly pan down until I got back far enough that I found framing that wasnt rusted. Be careful with the belly pan. I found that mine was very brittle in some areas.
Pictures of the area that you are concerned about would be a great help. There are many threads on this forum concerning belly pan. Pictures and questions will get you lots of peoples thoughts and recommendations