Is there a lot of weight behind the rear axle? Like tile floors, newer/ heavier bed, a big TV, filled water or propane tanks, or anything that wasn't there originally that would have added a significant amount of weight? That could very well be the cause of your wandering issues. I pulled a large trailer once, with a heavy tongue weight, and it wandered so bad, I had to stop and back the car up on the trailer a bit to take the weight off the front. After that it handled like a dream. Too much weight on the rear can cause the rig to wander pretty significantly.