The fellow who bought our 26 ft Avion last fall
which we hadn't used for over 15 yrs found some softness in the bathroom and kitchen. Since the original ply-foam laminate was of odd thickness he decided to replace the entire floor.
Since this trailer was built before the gray water laws, I had made the newly gray conversion about 30 yers ago. My tank I built of 3/8 marine ply with a bladder of 3 mil black poly sheeting of the Visqueen brand. Before installing the bladder I added 3\8 ply doubling strips from scrap around top and bottom for strength, then gave the entire box several coats of Verithane polyurethern varnish that was available at that time. For the bladder I folded the corners of the poly sheeting nicely and glued (with GE silicone) and stapled the poly to the doubled top of the box, as it was at that point and trimmed the edges.
The bottom of the top was likewise glued with marine grade silicone stapled around the edges. When partially set up, the top was g;ued and screwed to the box using stainless wood screws.
I go through this because after more than 30 years this tank was still in great shape. The new owner pulled the tank, tested it by filling it with water, added a 6-inch access at a logical place in the top for future flushing and cleaning, sanded the exterior, filled some gouges on the bottom and gave a new coat of polyurethane exterior varnish. We were both amazed how well that tank had performed over the many roads and years!
So the new owner, the biggest Asian Vietmese fellow I've ever known, and an excellent craftsman was happy to invest in marine grade ply for the new floor. After cleaning the frame, replacing some rusty parts (he's really accomplished with a small wire-feed welder) he PUR-15'd the entire frame, then coated it with several of black poly.
The new floor is a ply/expanded poly/ply sandwich as close in thickness as the original as he could make, but made with better materials and better construction practices than original. His wife, a beautiful Samoan, very proficient with a sewing machine as well as a table saw, redid the curtains and modified the twin aisle beds and front couch/bed appropriate for new futon mattresses all in a southwest Asian Polynesian motiff.
Unfortunately I was having severe health problems when they brought it by to show us. They are temporarily working in North Dakota and use it to supplement their small apartment. They also pull it with an upgraded vintage Ford pickup of the same year model, 1970.
So with the better materials available today any project like this is only constrained by your imagination and abilities. Good luck.