Liquid EPDM should work. Most RV suppliers or home building centers will have a form of coating like this one.
I did the entire roof of my truck camper 2 yrs ago with EPDM pond liner from Home Depot. Cost less than $200 to buy a 15 ft length 8 ft wide and glue it down to the roof structure. No leaks so far, but I will be coating it with the white liquid this year, just for UV protection.

http://www.epdmcoatings.com/whyepdm.html

I want to die like my Grampa, peacefully, in his sleep. Not screaming in terror like his passengers.