Welcome to the forum, Dave!
The roof life expectancy is dependent on several factors, how it was cared or not cared for, what type of environment it was subjected to (direct sun, tree sap, pollutants, etc.). Some of the rubber roofs have a white outer layer, then a black layer. The white layer will eventually wear off exposing the black. There are several roof coatings on the market, I have never used one so can not comment on that. I have read where these roofs can last 20+ years with the proper care. I would get up on a ladder and inspect the roof to see its condition, or get someone to do it if you are not comfortable climbing up there. In all candor, the roof should be inspected before the purchase to check for possible leaks, soft spots, the condition of the sealants and vents, and other possible problems.
Perhaps others will chime in on any roof treatment options. Good luck and enjoy your new Jayco!
2004 Chevy 2500HD Crew Cab 6.0l, 4.10r
2010 Jayco Jayflight 28BHS
Reese Strait Line with Dual Cam
P3 Brk controller