My old Mallard class A would do that with even a small crosswind. If the winds were calm it rode down the interstate straight as an arrow. I got use to it over the years and stopped noticing. I never did figure out why it acted that way. For the first year or two it really scared me. Then I realized that the white knuckling wasn't helping. I learned to relax and just drive it. With that said, when it was doing that I really had to stay on task. I kinda believe that it's a combination of things going on, suspension and rebound and a slight bit of slop in the steering. Mine was on an old P30 chassis.