You meant the roof vents that have the crank up lid, 14x14 inch opening?
They usually , now days, dont use rubber to seal them. If it is rubber you want, then there is a window butyl tape that might work. Any RV place should have this. $10 or so for a 10-15 foot roll.
Lap sealant is the most common, and easy to find at RV store, or in the SW probably at a hardware store even. clean the old stuff off, and put the Lap sealant down like caulk. The self-leveling will flow a little and level off and fill the crevices to some extent.
Eternabond tape will also work. It is a very sticky, almost like Silly Putty, tape that will stick and stick. Is removable with a heat gun, or just by pulling, but it will stick as intended for years. Get the 3 inch wide for a roof vent.
All the roof holes (vents, plumbing vents, antenna etc) will need to be looked at every 2-3 years. Maybe the current seal will last a long time, but typically after 2-3 years it starts to crack and let water leak in.
The same goes for the outside corners around the camper and all the wall holes like water heater, doors etc.
