I just read this whole thread - wow. It looks like the only issue(?) Dunnlnn didn't address, was the different tire brands on the rear axle. I wonder if changing that would have made an improvement ?
I got a new MH a couple of months ago and it too wants to wander, and that is without towing anything. I have to get it weighed, dial in the tire pressures, and possibly get a wheel alignment, although it tracks steady. I drove down the Keys a few weeks ago and the unit pulled to the right the whole time. It hadn't done that before or since, so I attributed that to the wind?
Nothing serious, but it's there (the drifting). My 24' MH on an E350 rides like it is on rails in comparison.