This is just my experience, we have a 21 year old 5th W. with the original EPDM and no coatings added. I keep it under a roof when not in use and wash it and put a rubber roof conditioner on it every other year. It is in very good shape for it's age.
Our 2005 TC had the Dicor coating on it when I bought it in 2012. It has held up very well.
After, I am guessing about 50K miles, it had a few scratches in the coating, not the EPDM, so I recoated it last year.
I have replaced the EPDM on 2 of our previous campers and I think that one would have been an excellent candidate for the coating rather than replacing the rubber. I had no experience with it at the time.
I will use the coating in the future as long as there is no damage to the decking.
You do need to clean it well and use the Dicor cleaner/activator so that is sticks well. 1 gallon was enough to put 1 coat on our TC. If it were the first coat I would have put 2.
The most time consuming part about using the Dicor was putting plastic over the sides to keep the cleaner/activator off. I don't know if it will actually damage the fiberglass but I didn't want to take the chance. It was in the spring but was too hot and the tape kept coming loose. I think it took me about 6 hours as I had to keep climbing up and down from the roof and I had no helper.
Good luck.
Joe and Evelyn