I'd start by checking the toe in with a tape measure and a friend to hold against a rib on the inside of the 22.5 tire near the tread. Want 1/8" toe in. Weighing and setting air pressures is good and maybe someone with the same rig will post their pressures until you can get it weighed... hopefully all for corners.
I agree with 45Ricochet that it's easy to over-steer a rig this long compared to a PU. Often it's the nut behind the wheel of a DP that's the problem... less so for someone trying to drive a gasser Class A. You have to give smaller steering inputs longer to take affect... easier said than done but with some practice it's often just a little more pressure in the direction you need to go... hardly any movement of the steering wheel. Windy days can be a challenge and easy to start over steering it, but here again with practice it's amazing how little input it takes.
Before spending any serious money, work on not over steering it.