Here's my "opinion" on the situation, for what it's worth....
If the decking is lifted from the underlying truss, and I was looking for a permanent fix, I'd remove the side molding, pull back the TPO membrane, "GLUE" the decking to the underlying truss and use screws (not the factory staples) to secure the decking to the truss. After that is done, I'd leave the TPO open for enough time for the adhesive to cure and harden so any "outgassing" wouldn't harm the TPO membrane. Then reseal and reinstall the side molding.
The reason I'd do it that way is because screws through the TPO are probably going to back out with the vibration/flexing in the joint during towing. Usually the factory glues the OSB sheeting to the trusses and shoots staples (often far too many) into the structure to hold it in place until the glue dries. In the "hurry up world" of RV assembly, often the decking isn't dry before the TPO is applied and when pulled tight, the TPO (not the staples) pull the decking into position on the curved trusses. Once the trailer sits in the sun for a while, the TPO gets warm and stretches enough to allow the decking to lift slightly. If you really look closely at almost any RV, you'll find gaps and loose sheeting on almost every RV roof decking.
If it were me, I'd leave it alone if it's "a loose OSB sheet that's lifted a quarter inch and I can push it back down".... It's under the TPO, screwing it back down isn't a permanent repair, putting DICOR sealant on top of screws is a "waterproof fix" only as long as the DICOR is intact. If the screw starts backing out (and they probably will) the screw heads will push up on the DICOR, lifting it, stretching it and possibly breaking the seal at the edges. If that happens, you've got a direct path for water (under the DICOR, to the loose screw threads and wicking directly to the OSB/truss wood.
I'd leave it alone if it's "just a loose decking sheet" and watch it. As long as there's no damage to the TPO and no staples/screws pushing up on the TPO, I'd not attempt a temporary repair until a permanent problem is imminent.....
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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