This is a repair I did years ago on my old Thor TT which I did a paste & click from another camping forum. The floor was a lauan/styrofoam/lauan sandwich type
This leak could be from condensation, Years ago I did a search on the internet when I had the same problem with mine TT and what I found was the moisture wicks up to the top of the floor and if uncorrected will rot under the linoleum.
Two things that I did was pull up my linoleum in the front 1/3 of my TT at the end of the camping season and let dry over the winter. Then I sanded the old floor in the spring, put down liquid nails and laid the new floor on top. I also put in wood screws to help secure the new floor onto the old and where my new floor was under my seating I also drilled through the floor and fastened 3/8'' x 4'' ( If I remember ) nuts and bolts around the perimeter of the floor for more extra strength.
Second thing I did was to cut out patches of the aluminium underbelly with a dremel cutting wheel and replace with bug screen. This allowed the underbelly to breath and stay relatively dry. I got this tip from a reputable RV mechanic that I know.
2022 F-150 3.5 EcoBoost 4X4 Supercrew GCWR 19,500 157WB
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2017 Jayco Jay Flight 24RBS
Old TV, 07 Toyota Tacoma, Double Cab, Factory Tow Pkg, retired towing at 229K. (Son now owns truck)