I just replaced my roof vent in my 85 Bigfoot
with a Fan-Tastic vent 6600. The RV place here recommended the Dicor lap sealant and I am very happy with it - sits down nice and adheres well (ok, something along the lines of "...to a blanket"). So a touch messy (there's some skill involved).
I had to remove many years of dried out sealant around my old vent. I found it was quite brittle, but still adhered well. I used a 1" wood chisel to scrape it off (a LOT of work) and got back down to the clean fiberglass surface.
So you could choose to either remove all that is there, or seal over top of it again.