If there is a soft spot that usually means that the roof decking, which is usually OSB or particle board or plywood, is rotten. It can't be fixed from the inside. But poncho62 is correct-you need to find and fix the leak to stop the damage.
I just went through a roof replacement due to an accident involving a tree limb. The entire rubber membrane was replaced along with one 4x8 panel, the satelite dish, 1 roof vent and a plumbing vent. Everything on the roof was removed and reinstalled. This was done in a shop that repairs RV's and heavy trucks/trailers. They also installed 1/8" luan paneling over the decking for a smooth finish as pulling off the rubber membrane from the original decking leaves a rough finish. Cost was around $9100 which included about about $600 in damaged parts and $350 to clean the ceiling due to water staining. Labor hours were 58 @$110 per hour, the biggest cost of course.
It's a very expensive repair but if you really love the unit replacing the roof may be the way to go. You may not find a better unit.