I used a thick rubber mat that was designed to fit my truck bed. I would never consider using a plastic liner on the painted bed surface. IMHO two smooth surfaces could have been trouble. My Lance sat on the mat and did not move at all. I removed the mat when I hosed out the bed area.
P.S. Several years ago a coworker mentioned that he left home with a plastic bedliner in his truck and when he arrived at work, it was gone. He didn't see it fly out of the back of his truck.
2005 Winnebago Sightseer 30B, F53 Chassis