I had a plastic liner in my old truck which was a 95 Sierra, I looked under the liner before I sold it and while some paint was rubbed off I saw no bare metal and no rust or moisture under it. The liner was in the truck since new FYI.
My new truck has a spray in, I like it and it's nicer but I don't make it a habit of dropping things in the bed so denting isn't a concern I have.. The diy Wil never be as good as a commercially applied liner.