My old camper had roof leak issues. I used the following material to re-roof the entire camper and it worked VERY well.
OneStep | ER Systems
It is elastic, approved for application over EPDM roofing, pure white, does not chalk, run or mildew. this same material is used by the Burger King stores in the SE USA because of its durability and since it is bright white they save on their electric bills in their restaurants.
I purchased it at my local home roofing supply house. Less than $600 for a 5 gallon bucket and two tubes of the compatible caulking sold in tubes. That would more than cover any RV up to 40'.
I WILL be using it again on my new unit as soon as I get any cracking problems with the lap sealant just to INSURE i don't get any water intrusion from my roof.
NO, I do not own stock in it or have any affiliation with the manufacturer.
Just a thought