underbelly lining no longer attached

That is interesting. I had issues with a wet floor bottom on my 2015 178. After getting the pod out of the elements, I traced mine to a leaking spoiler. I allowed it to dry, and then Eterna bonded my spoiler ,top and sides. This is my first season with the new seal. Appears floor is staying dry, I'm talking underneath. No apparent issues looking at it from outside at camper level. Also rear pass thru was not wet. I didn't lose any bottom side lining, it was wet. Nothing apparent inside of Pod. All the moisture was on the underside at rear, in my case. After you have deduced where the moisture is coming from there are several good sealants you can use on Pod bottom. Noting will work until the water issue is resolved.

Good sign. May have just lost adhesive capability. Maybe. When dry, recoat. You could use a spray on or a brush on to test your theory.

When I have repaired the bottom seal and I admit probably not to the extent of your issue. I used Flex Seal, black, in both the spray and brush on types. I've noticed most of the belly hardware is attached with self tapping screws into the non metal bottom. When I replace or attach something on bottom I recoat the area with flex seal.

I wonder if textured 2-part pickup truck bed liner, that you apply with a paint roller, would work?

The black rubber underbelly cover has now been 60% removed. Cut out with a knife. The thin layer of wood is drying. Foam above it seems dry. Need to replace the cover or coat the exposed wood with something. Could not pull the cover off where it goes under the frame or under the tanks. But still is drying out and not rotting. Leak was at seems between rubber cover and wood layer at the bottom underside edges. Front edge, side edges and rear edge all allowed water to seep in between this layer Of the floor. The only way the wood would ever dry was to cut out the rubber underbelly stuff which was delaminated from the wood. Kind of like a rubberized nylon mesh now 60% removed. Thanks for your help. I’m in Canada and 1gal cans of flex seal are unavailable where I am.