I have a 1999 Dodge Roadtrek 190. When I bought it I replaced the old tires with a set of Michelin LTX M/S. The installer put 45 psi in the front and 80 psi in the rear. The next day I started a two hour trip to Jacksonville, an hour into the trip I was so frustrated from fighting the wandering issue I had decided to sell the Roadtrek! Thankfully, upon arriving in Jacksonville, I did an Internet search and came across some posts from Red Rover. He recommended putting 65 psi in the front tires. I purchased a quality tire gauge at the first automotive parts store I found, after finding a station that stills pumps air, I put Red Rovers recomended 65 psi in the front. The wandering disappeared!!!
I have put 5000 miles on my much loved Roadtrek since then on every type of road without any wandering issues. In windy conditions it does move around, but I consider it normal for a tall van to do so. You may have other issues with your van, but for me, the combination of tires and air pressure made the difference.