There could be a lot of dirt and other garbage under there. My normally white bed was almost completely brown when I pulled mine off. Same thing, previous owner put it on and never touched it again.
Depending on where you live, I would pull it off and make sure there is no rust under there. I got lucky, this truck has always been in New Mexico where rust is pretty rare. I would check soon.
As far as bedliner goes, I'm planning on installing monstaliner. Heard great things about it, and the price is not that bad