It isn't difficult. Well that is a lie... it's a pain to get under a low slung Airstream and then try to work. Specially for the rotund.
So, run both axles up on a ramp. You'll see that the "belly pan" is nothing but a couple of sheets of galvanized metal held on with screws that have a big washer under the hood (some units have rivets, not screws). Drill the rivets or unscrew the screws... then try not to breathe as gravity dumps the whole mess on your face. (seriously respirator or mask and goggles - even on a new one there's going to be dust and dirt, plus a spider carcass or two.)
You may have newer styrofoam under floor insulation rather than the older fiberglass.
Getting it back up is usually the problem. Viagra? NO we're talking about the BELLY PAN! If you don't have about six hands getting the pan back on can be challenging. I used the plastic leveling blocks, several boards, etc.