If you can build a dog house, you can repair a soft spot of that size on your trailer. It will take an afternoon of your time.
If you're really hesitant about doing the carpentry, visit the big box store and buy some Git-Rot or similar epoxy for rotting wood. Apply it to the soft wood, let dry, and roll the roof membrane back into position, using some adhesive. When you have the membrane back on, cover the seam with Eternabond Tape.
If you need a break in this process, tape a garbage bag onto the membrane across the work area. That will secure your rig from rain until you can complete the repair.