Has anyone ever cut out a section of their trailer's under belly to gain access to plumbing, etc.? If so, how did you reattach it? The covering on my trailer is a sheet of flexible rubber.
If you needed a smaller hole in between the frame rails you could take some wood and make a picture frame of sorts around the hole splitting the wood between the existing edge and the opening, use drywall screws to attach the wood to the panel then put the piece you cutout back in the opening and screw that to the wood that overhangs, then tape edges with tape. The wood will help stiffen the opening where your cutout is and also lets you get back into that area if needed.