On our ElkRidge 34QSRL, when we decided to add heat tape and heat pads to our tank and pipes, we determined there was no easy way to get into the underbelly without cutting. The spare tire, as well as the rear stabilizer jacks, are secured *after* the underbelly in in place. (not to mention the gas line running 2/3 the length of the rig.) We cut ours in two places (in front of the rear jacks and behind the spare tire (located at the front of the rig for us). We also had to cut around the drain pipes and low-point drains, as well as cuts near the brake wires. Getting the underbelly out from around the axles was the hardest part.
We also had quite a time wrestling that back into place, and taping was a challenge, as well. We cut ours right at the cross supports. We used a thick, window flashing tape (about 6" wide) to tape ours back.
Click here for similar. 9 mo later, still holding.
For the work you mention, I am not sure they could have avoided some cutting around the pipes, but probably saved you "billable time" doing it the way they did.
Erika