Most EPDM and TPO roof membrane systems are sold with a 10-12 year warranty. Cars are sold with a 36000 mile/3 year warranty. Televisions are sold with a 1 year warranty.
None of the above are "designed to be replaced at the end of their warranty period.
Most RV roof manufacturers have a "sales pitch" in their advertisements that says something like this: With proper care, our roof system will last 20 years or longer.
It sounds to me like you have someone looking who is "astute at negotiating"...
Here's what Trailer Life has to say about EPDM roofing, "Although most rubber roofs are guaranteed for 10-12 years and can easily last for 20 when well cared for..."
http://www.trailerlife.com/tech/diy/...-tiptop-shape/
DICOR, the manufacturer of most EPDM roofing material during the time your trailer was built, has quite a bit of information on the internet. They also suggest that RV roofing, properly cared for, will last much longer than the "warranty period".
It really depends on the condition of your trailer, how eager you are to sell it and how many buyers you have "looking"... It sounds like this guy is interested, but trying to get you to lower the price using "rumors and bargaining"..... Up to you whether you agree.
ADDED: This statement from DICOR is in their FAQ section:
How long can I expect my roof to last?
If properly maintained, the roof should last a minimum of 20 years. The membrane itself has been weather tested for 20 years.
https://dicorproducts.com/faq/