Cut a "U" shape access just underneath where you suspect the line is. Make the "U" cut large enough for you to have access and work.
Make the cut so the upper part of the "U" (the open part) is facing toward the front, the uncut part is facing forward and will act as a hinge. Pull down on the cut for access. When you are finished, use a tape that is made for RVs, 2" wide and black or gorilla tape. The cut can be closed by placing tape on the top of the cut with 1/2 of the tape (sticky side) facing down. Simply push up on the flap (CUT) to make contact with the tape. Use a "SNIPE" sign procured from a local street corner (That advertises "FIX YOUR CREDIT") Use the snipe sign cut to size to place over the cut to amply cover the cut in the underbelly, use small sheet metal screws (deep thread screws) to secure the "PATCH" over your cut.
Place the screws about 4" apart. Tape the patch to form a good seal.
Airborne, if you can jump out of a perfectly good airplane, you can accomplish the above.
I re-visited your pictures and the information above may not apply to your RV. It looks like the "UNDERBELLY" is a soft material of plastic. But the tape I advised using is good stuff even for your underbelly!