Sealing up the underside of a Fleetwood or Coleman would go against what Fleetwood and Weyerhaeuser (actual manufacturer) say for the Structurwood used. Sealing it would actually trap moisture in the wood.
I quote the 2008 Fleetwood folding trailer brochure:
"Weyerhaeuser agrees with Fleetwood's practice of not coating the exposed bottom surface of the Structurwood panels used in their trailers. In our opinion, moisture from occupancy load, or from other internal or external sources may become trapped in the Structurwood substrate if it is coated on the underside with a system that impairs the ability of the substrate to dry thoroughly. Trapped moisture, over time, may cause premature decay or rot." Weyerhaeuser
I had a Jayco that had OSB wood for the underside. I don't know if it was Structurwood or not. It had a coating on it. Several wafers had peeled and warped. You could tell it had not come that way from Jayco as there was no coating underneath the wafers that had warped. I am left with only the conclusion that water was the cause and might not have had that effect if it were allowed to pass through unimpeded.
I don't see anything wrong with putting a breathable membrane underneath but I sure wouldn't seal it up.
