O.K., here's my thought. The fact that it's a "salvage title" is irrelevant here, isn’t it? The real question is “Will a bed liner product prevent/protect against any rust?”
If that’s the case, my perspective is that if there’s rot all the way through, you’re going to have to do some cutting and welding. If there’s places where you can’t put weight on the floor board, then you’re not going to be happy. The lining, whether rhino or another brand, is a coating not necessarily the material of construction.
If the steel is in good shape, after you scrape off any rot, then you'll be able to coat it.