I had a similar situation. I discovered a soft spot after having a drip on the bed on a couple of occasions. It turned out that my vent cover had made a small cut in the rubber roof. I cut the rubber a few inches larger than the piece of OSB I was going to have to replace. I peeled the rubber back and cut the bad piece of OSB out back to the nearest seam in the OSB. I added some reinforcing where the new seam was not over a rafter. I then re glued the rubber to the roof and used eternabond over the seam in the rubber. The hardest part was finding the correct thickness of OSB. It is not a hard job if you have any carpentry skills and a few tools.