Not simple at all on a Trillium
. The Trillium design for the 1300 and 4500 models, all the interior cabinets are fiberglassed directly to the shell on the sides and top, and directly to a fiberglass floor on the bottom. Under that layer of fiberglass you have plywood. Under the plywood you have the outer fiberglass shell.
Trilliums are designed with pontoons around the perimeter, which give water leaks
a chance to avoid the plywood. But if you rot the plywood, now the work begins.
Dave Tilston has recommended in that case, remove the body from the frame. Then carefully roll the body onto its side. Then cut the outer fiberglass shell on the bottom. Now your plywood is exposed. Remove the rotten plywood, reattach and fiberglass in the bottom shell, roll the body back upright. Jack it up. Put the frame under it. Reattach body to frame.
Now the rear dinette has a raised floor, it can be pieced in, still a chore, but not as bad as the main floor.
I looked at two Trilliums that both had main floor rot. So its very possible.
The plywood on the floor extends past the edge of lower cabinets: kitchen, closet, and gaucho. The frame is such that it does not lend itself to piece wise repair.
If you cut the outer shell at the bottom, you are cutting into the pontoons.
The toilet up front may give water a much easier path to the plywood.
If this sounds like something you are up for, god bless you. Be sure to take a ton of pictures of the repair process and post a thread here. You will be navigating new territory.
The lack of rivets makes the Trillium interiors very nice. The lack of rivets also makes removing interior cabinetry infinitely harder.