No clue what the advertised life expectancy of the typical RV rubber roof is, however I have helped several of my guys replace them with high end commercial/industrial stuff that has a life span of 30+ years.
Helps that we are a professionals
We have one guy that works for us that is still hauling a 15 year old high end fiver and probably will for several more years. If you purchase a gently used high end fiver and do some strategic upgrades you will come out pretty well over the years. I know that my Airstream will eat up close to $10,000 before I am done, but that is a lot cheaper than a new one and should last me until the end of my days.