Dicor lap sealant (self-leveling on horizontal surfaces, non-self-leveling in vertical or contoured surfaces) is the most common sealant used for RVs with rubber roofs.
However, as UncleBilly says, there are companies like RV Armor
that will do a permanent, spray-on roof coating. The spray-on coating is something like spray-on pickup truck bed liner except it's white. The spray-on coating is expensive, but it is maintenance free.