Just finished fixing all our Trilly's leaks
and learned that they all cannot be blamed on the leaks
from the belly band.
Yes, the belly band was leaking, but there were also leaks
from around the frame of a window, holes in the side of the Trilly, from rivets in the awning
strip, from screws of the door (from down dripping water), etc. The other sources of leaks were discovered after the belly band was caulked and leaks persisted. Good thing I didn't attempt to solve the leak problems by removing the belly band and fiberglassing the gap.
The rubber strips and weather stripping of the top edge of the belly band did not seem to fix the leaks, so I went to caulking the top edge of the bellyband with Sika 295-UV ( low strength polyurethane) used for windows
The holes in the fiberglass walls were plugged with Pro glass fiberglass filler or West Systems G/flex epoxy.
Rivets and screws were plugged with Capt. Tolleys Crack Cure and Parr Parabond clear.
Window frame edges were sealed with ProFlex clear caulking.
Holes in the avocado green interior were painted with Behr Ultra Natchez Moss which perfectly matched the color but not the original glossy finish.
One of the advantages of living in the Pacific NW is that it acts like a laboratory for your trailer to test it for leaks; there is no need to drive to a leak testing center like the Airstreams frequent.
All the caulking jobs required hrs of prep time, cleaning with isopropyl alcohol repeatedly, using other recommended activators and primers, taping with 3M low adhesive painter's tape, using a good quality caulk gun, making sure the caulk was not cold, having an assistant to put pressure on the caulk bead while you caulk, then pull the tape ASAP.
Also, be aware that caulks and primers all have shelf lives; caulk expires typically in 12 mos, so be sure to check the date on the tube. It was possible to caulk the entire 30' belly band top edge with 1 18oz tube of caulk, but a second tube was available if needed.
Hope this helps. Good luck!