I rebuilt an old camper that needed the roof vent replaced. I used an RV lap-sealant by DiCor. It is self leveling so floats into the nooks and crannies. Seems to be readily available at most RV places. It comes in the usual style caulking tube. Two year on, exposed to the winter weather and West Coast rain and still sealed up.
Take off the vent and scrape and clean the surface. Buy a new vent (they are cheap) cover the flange in mastic putty and screw down. Liberally cover the joint and all screws with Dicor caulking (dont spread it as it does it by self leveling) Scrape and clean all gutters and trim that is horizontal and caulk with dicor ($10-12 a tube) as well. Dont mess with anything you buy in a building supplier as it wont last.
It is a tape. Comes in different sizes. I used 4 inch wide to seal all of our roof vents. I also bought it off Ebay. It is expensive in Canada but a bit cheaper in the USA if sent to a post box. I had them send me a sample before I used it. About the only thing it won't stick to is silicone.
Eternabond tape 4" x 50' $98 and change Amazon.ca
Proflex RV .Nothing works better. Sikaflex is a waste of money compared to Proflex.
I am in the process of doing some resealing with EternaBond. Goes on pretty easy and appears to be pretty forgiving. Word to the wise, only peel off the plastic as you go. Once it sticks to itself, ain't nothing unsticking it. Ask me how I know.
Designing the perfect tin foil hat.
like alot of people said pull vent and clean all old caulking, putty etc. Put new Butyl putty down and a new vent, screw it down. Then for ultimate fix use dicor caulking then after caulking is cured cover with Enternabond tape. If you want to save a bit of money just cover vent with enternabond tape and no dicor caulking. I just did my fifth wheel with enternabond tape and Koolseal. I now have no leaks from the roof. I bought all materials from Amazon.com and shipped to Blaine. I think I paid 25 dollars for vent and 55 dollars for 4" x 50' enternabond tape.