I would re-post this ? in the "Vintage RV
" section of the forums. That's a popular / on-going repair request. Now I was at the RV show here in Cleveland yesterday, and one of the vendors was selling a service where he put on a 4 step process of a rubberized roof. It was 4 coats applied over what ever type of roof material you had. His company was called RV Roof Shield LLC. or RV Roof Shield
The material after the 4th coat was about 3 times as thick as the rubber (EPDM) used on new trailer today.....