Depends on your ply. If your ply is a bit of a mess then yes pva it as the pva will help seal it. But if your ply is new or in good condition then you have no need to seal it as a good contact glue will adhere to ply no problem. T:
I see I thought you were worried about you carpet sticking. I've never worried about moisture absorption. We have been camping in all weathers for 4 years in our van and it's never been damp. I didn't pva mine bud and I've had no problems what so ever. That said pva is cheep enough so slap some on if you have time it won't do any harm.