[JRTJH]

When I first read your post, I was thinking condensation/mildew between the cushion edges and the exterior wall. Moisture will condense from the warmer humid air and collect on the cooler outside walls, especially where there is no air movement (behind cushions and mattresses). But, the further into your post I got, the more it sounds like a window leak. If it was bad enough to need to replace the cushions, you may have mold established in the OSB/plywood framework of the dinette. If so, you might get some help from your insurance company.

I believe the Bullet Premier slide walls are vacuum formed. If that's true, then any holes you cut will only reveal "foamed in place Styrofoam paneling"... So, possibly a better approach would be to remove the side window at the dinette and see if you can identify any water intrusion from there. If you're careful, you can see if any water has encroached the luan paneling under the window framework.

When you reinstall the window, use butyl putty tape and be sure to tighten the interior trim ring evenly around the circumference as you go. Don't try to tighten the top then move on or you will warp the frame, tighten gradually as you would the lug nuts on your wheels.