They make rubber roof paint that basically covers sins. It can be a one-way street, though. When I bought my RV the roof had already been painted completely with the stuff, covering all fasteners. It is basically impossible to remove and do work on the roof anymore.
Once you reach the point in life of an RV where roof replacement is not financially reasonable, about all you can do is keep slapping on roof paint if a leak shows up and keep going as long as you can. And then, stay off your roof, because it may not support you anymore.