I would agree based on the shared pictures, the ratio of wheel base to rear overhang seems excessive. But I wouldn't give the rig a bad rap. I think you can resolve your problem without breaking the bank or selling the rig.
You on the right track having the right things added, but simply stopped short in what is needed to eliminate the wandering, or at least greatly reduce it to the point that you are satisfied.
I feel the following should be considered.
- a rear trac bar (highest priority)
- Koni-RV or Bilstein's heavy duty versions shocks all around
- front heavy duty stabilizer bar
About that wheel alignment you mentioned.....It MUST be done as if loaded on a trip with full fuel, full propane, full fresh water, and empty waste tanks. Even you personal belongings and non-perishable foods and such.
When I took ours in for a wheel alignment, I even placed exercise weights on the floor just behind the driver and passenger seat to consider people sitting in those positions. It all effects the wheel alignment.
Without weighing the rig, it's hard to say what tire pressure to consider. But based on the over-hang, I would think your 80psi in the rear tires is right-on. You might want to try 60psi in the fronts.
Well wishes. Keep us updated.
BTW: We had similar troubles with our 2007 E350 rig. I went all-out and did as I advise for you. All better since....handles dreamy.