Assuming your front end is in good repair and adjusted correctly. How are your tires, shocks and steering dampner? Basically you are driving a huge sail and your going to get blown around to a certain degree. Make sure your MH's front end is aligned and make any necessary repairs. Worn tires and steering dampner can really make your rig squirrelly. Change to radial tires and get a quality steering dampner such as Bilstein or Safe-T-plus. Worn shocks can also aggravate the side wind issue. In general stiffer shocks with harder damping will help. Again Bilstein are probably the best out there. Last but not least, checkout a Steering centering system. Units from Howard power Steering systems, Supersteer and Steer Safe. All claim great things, check them out at your local camping world. Remember all the little things add up. Front end a little out of align, worn tires, shocks & dampner and you can have a big affect on handling.