I have exactly the same thing happen in my Chevy Roadtrek. This happens on roads that have uneven surfacing. The front hits the bump and then the rear and it is like riding a bucking bronco with the van pitching up and down and up and down as it hits these bumps along the road one after the other. The best thing to do when this is happening is slow down. If I am driving the speed limit at 65 it is really bad, but if I slow down below 60 it gets better - doesn't stop, but better because there is more time between the time the front tires and the back tires hit the spot. It has nothing to do with the tire pressure. It is the road surface. Better shocks may help (my shocks are new with the 2011 B and it did this from the start). I have seen RV add ons advertised for larger Class MHs that say that they address this problem. I am not sure how well those will work on a B. The wheels are close together, the van is tall, and the van is heavy. In a small lighter vehicle you would not even notice this - I have driven the same stretch of road in my B, in my car, and in a passenger van and while you can feel going over the bumps in the car and passenger van, they do not pitch and bounce like the B does.
