RV Underbelly Tape 4" x50', RV Underbelly Material, Waterproof Permanent Sealing Adhesive for RV, Camper Travel Trailer, Mobile Home, Tear Repair Patch

Cut a big 'H' into the underbelly, that brings 2 flaps down and makes it easy to put back up with either a small piece of luan with some very shallow screws (put the wood above the underbelly and screw both flaps to the wood and watch out you don't pierce the tank!), or with tape designed for this purpose, which is called 'darco tape'. https://a.co/d/0hHC8iLM For example.