I'd like to see a side view photo of this coach. The ones that look like the House (from back of cab and not including front overhang) is balanced on the rear axle seem to be the ones with the most handling issues.
From the thread, it seems unlikely that the front axle loading will be found to exceed about 1/3 of total scaled weight (front plus rear). If this chassis is E-Series, the front axle tire inflation is only 65PSI at FULL load. Yes, the rear tire pressure can be the full sidewall number of 80 for a fully loaded E450.
Overinflated FRONT tires don't have enough of a contact patch with the road surface to care if they go straight or wander.
Make sure both front tires are at the SAME pressure and that pressure's no more than 65. I personally use 60. And that all four rear tires are at the same pressure. I use 70.