I found where the leak is yesterday. There is a iron pipe that carries the gas across the chassis to the hot water heater, and one of the joints was leaking. I was afraid that the joint had split, but the sealant apparently worked loose. I had to take the connection loose at the hot water side (copper to iron), then loosen the pipe, and put fresh thread sealant on the pipe, which fixed it.
I am waiting to hear back from the dealer about the slide, but have already figured that using a sheet metal patch; shoved into the L bracket along the base of the side, and extended about a foot or so up the side along the fiberglass would work. I would make several rows of screws to distribute the load of the couch more evenly on the side, then paint the patch on each side. I believe this would be a better solution than just replacing the sides, since the faulty engineering would persist if the fiberglass is simply replaced. I am going to see if the factory has a suggested fix (and offers to fix it here). Otherwise, I plan to fix it myself.
For those asking, I am feeling pretty darn good!