I'm a little confused about having the windows installed backwards; that's really not possible. Upside down, maybe, which would definitely result in a water leak. In any case, it is apparent moisture has invaded the interior walls. If indeed the window leaks have been stopped, it's possible only relatively minor repairs are required. But it will take an expert RV service technician to fully inspect and assess the resulting damage. Some of it may be hidden inside the wall. In minor cases, the delamination of the exterior surface will often cease once the leak is sealed. But if the interior paneling is weakened or rotted, the problem may be more severe. In most cases, the damage can be mitigated. How much it might cost is unknown until the inspection process is completed. I would definitely find a qualified service tech to fully inspect the damaged area. I do have an experienced, Master Certified RV Technician in your area I can refer you to; contact:
Ron’s Mobile RV Services, Inc.
Serving Eastern Pennsylvania
He'll come to your door and perform the necessary inspections and to assure all leaks have been eliminated.