Novice here on issues like this.
Just thinking out loud here.
You are going to re-engineer the entire roof? They are going to remove the fiberglass roof and replace it with an EPDM roof?
I dunno, but that seems really extreme and really expensive. Maybe
overkill. I would really think hard about a project like that.
To be honest, is an RV of that vintage really worth sinking in what could be $5,000 - $8,000. Even after you spent that much money, what will you have as an end result? An RV designed for a fiberglass roof that now has a EPDM roof that may or may not actually perform properly. To my thinking, that is good money going after bad.
Just my opinion, and I really know nothing about your situation. Just shooting from the hip.