So heavy rain is your main concern. The plan should be a careful inspection of the roof. Get up there and look very close at every square inch.
The most likely places and the first leaks to appear are at seams, edges, around roof vents, air conditioner, and other fittings.
Next go over the "rubber" surface closely looking for cracks or tears. If there are no "rubber" cracks or tears any fix will be easy and low cost. Use DiCorp self leveling sealant on any cracks in existing sealant. It comes in caulking tubes. I had to buy 6 tubes even though I only needed a couple of ounces, but it is the best for sealing over every crack.
There are other products and they may be good or bad. There is EternaBond tape for RV roofs. I only know about the DiCorp self leveling. Here is the DiCorp website where you can find instructions for use. You can buy it from many on line RV stores.
Don't use hardware store caulking and especially not silicon products. Silicon is to strong and will eventually tear the "rubber" at the edges of the silicone bead. If silicone or other caulking has already been used, I think it must be completely removed before resealing.
Google "RV roof repair" for extensive results.