Wow, your pictures bring back memories of my floor replacement job in our 2001 Palomino Stampede S17 HTT. We bought it used in the fall of 2009. It seemed like a good buy at the time. After I got it home I noticed that the floor inside the door seemed to slope down from the frame to the edge of the trailer:S. I suspected a poor seal at the door weatherstrip and fixed that, then found a crack in the wheelwell caused by a cracked frame shifting the wheelwell. I welded the frame and patched the plastic wheelwell then I lifted the vinyl floor and did a patch job at the door as that was the only area that seemed to have an issue. I did notice that the vinyl flooring at the very front end had popped up through the staples, but didn't suspect how bad the whole thing was until the summer of 2011 when a very soft area appeared in the area between the kitchen counter and bathroom door. I started to rip the old floor out at the end of the camping season that year, which meant completely removing all the interior that was attached to the floor except for the fridge. It looked very similar under the vinyl to yours. Our trailer had the same poly tarp material on the bottom of the luan/styrofoam sandwich, which along with the vinyl top, allowed all the water that managed to get past the joint seals, door seals, openings cut in the tarp for wiring, propane, water services etc., that were not properly sealed/maintained, got sucked up by the luan and started the wood to rot. Not having access to a factory to make a new floor the same way(one piece sandwich), I got 7 pieces of 3/4" 4'X8' good one side fir plywood and, starting at the front, jacked the trailer cabin off the frame far enough to slide the new plywood floor(2 pieces screwed together to make up the correct thickness placed crosswise)between the frame and the cabin. I staggered the joint at the back of this section to allow for overlap to the next 2 sections(one side at a time), which were installed lengthwise and joints staggered in the centre. I sealed all the joints with the best caulking I could buy and sprayed waterproofing on the underside of the new floor before installing it and made sure to carefully seal the openings of all services that went through the new floor. I also installed sheets of roll aluminum between the frame and the outside edges of both sides of the trailer(available at building centres for contactors to make trim and eavestroughs for houses-painted white on one side and brown on the other). I replaced the bottom edge trim with new pieces purchased at a trailer dealer. I then spray undercoated the entire underside to ensure water-tightness. I had a professional install new vinyl flooring before re-assembling the interior using all new screws. I replaced all the rusty exterior screws with stainless ones painted white. To look at the trailer now, since I did the job in the most professional manner possible, you would never suspect that anything had ever been done to it. We intend to keep this trailer for many more years, so scrapping it or selling it damaged at a loss was not an option after what we paid for it. It's good that you didn't spend any money on it, so can afford to spend a little to fix it up. Luckily I had the skills and tools necessary to do the job properly since I am a licenced senior master technician at a Ford dealership, as well as having worked at a trailer factory for three months assembling trailer walls and floors before I became a mechanic. At my shop at home I have a variety of jacks, metal working tools, welding equipment, woodworking equipment etc. The only drawback that I can see for using the 3/4" plywood is the extra weight that was added to the trailer. Luckily out TV didn't seem to feel the extra weight. Good luck on your project and I hope that it turns out as well as mine did. Make sure you inspect all edge seams for water intrusion, especially those along the bottom edge of the side walls to prevent repeating the repair at a later date.