[anoldman]

I'm certainly no expert on roof coatings, but from what I've read the rubber roofs that come on RVs now are far superior to the roofing found on older campers. The flexibility and resistance to cracking is the key. The bed liner coatings I've seen were pretty hard and inflexible and I doubt that would last long before cracks would form. Yeah, that roofing stuff is expensive, but right now I'd say it's your best choice if your camper is in good condition and you want to keep it that way.
Good Luck!