Update: I have managed to gain access to the insulation sections outside the frame rails behind the wheel wells. To achieve this I pulled the staples from under the lower edge of the aluminum siding, so I could pull the siding out a little. This allowed me to see the plastic sheeting, where it wraps around the edge of the floor joist and is sandwiched between the floor and side wall. Using a knife and straight edge (fabricated from an old hinge to provide an offset from lower edge), I was able to cut the vertical section of plastic sheeting at the mid point. After pulling the plastic down, I was able to pull out the wet fiberglass insulation (a stick helped at the ends of the cavity).
I was anticipating the insulation would be separate sections cut to fit between the floor support beams. However, it appears to be one large sheet that is installed across the floor beams before the plastic is stapled in position. When the floor structure is dropped onto the frame, the insulation is compressed where it becomes sandwiched between the floor beams and the frame. Where I have removed the insulation, this compressed section is still trapped between the beams and frame, and hopefully will dry out before too long. The fiberglass insulation is still present in the section between the main frame rails but the water ingress into this area is less severe.
In the areas outside the frame rails, I have now replaced the fiberglass insulation with polystyrene blocks that should not absorb water if the area gets wet again. I am now in the process of reattaching the plastic sheeting using 2" wide FlexMend tape. This will cover the slit cut in the vertical section of the plastic sheeting and will be completely hidden when the aluminum siding is put back in place. Other than some added sealant, the repair should not be obvious to anyone looking underneath the trailer (e.g. future trade in).
Next I will be looking at adding some corrugated plastic between the frame rails, to better protect the openings in the plastic sheeting around the holding tanks. Since the waste drain plumbing was not designed to accommodate this, it is going to be a little more challenging that it would be on other trailers. Overall this is turning into quite a project, all of which could have been avoided with a little bit of design effort and attention to detail during the build process. I feel the RV industry needs a wake up call, much like the auto industry had with the arrival of Japanese imports.
