If the underbelly is the fabric type, you probably have no choice but to cut it. When done, tape it back together with some really strong tape.
If it is the cardboard (black) stuff, it is screwed on in sections. Simply unscrew a section and remove. When done, screw it back on.
I had the fabric type and had to make an access once. I used the aluminum tape, used for taping aluminum stove piping to seam it back together. It held for years and was still going good when we traded campers. Since then, they've come out with different brands of equally strong tape that would serve for years and years.