I can only tell you what I believe to be the cause. All I can think of now would be do the most you can to dampen bouncing on the rear springs. It's new, new shocks, but are they HD and/or adjustable like Bilstein or Koni? Maybe install a full air-ride rear suspension? I can't guarantee success with either.
I'm not overjoyed with how Ford chassis like to wander on the road, and I'd like more cab room. Still, Ford allows more wheelbase stretch than GM does. I wonder if the coach/chassis combination you chose is limited in wheelbase by the GM chassis.
Another school of thought is that the interior designers place a certain rear bedroom design above what the overhang does to handling on the road. They need the wheel wells out of the way.
Letting form govern function is common but shouldn't be allowed. We've had a member trade a coach in because of issues involving overhang.