As mentioned if you know what area that needs attention a small cutout may suffice. You can reseal with Gaffers Tape or the Gorilla Tape... both are superior to regular Duct tape that over time will wither, peel and leave the goo. Bigger jobs may involve unscrew, unbolt of a big section. Sometimes where a pipe comes through you have to use a knife to trim the foam sealant and then re-foam when putting it back together. Maybe some of the Keystone Manufacturing Process YouTubes will give you a peek at the frame underbelly and how they put that big sheet on.
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