Everyone is providing sound advice.
If proper tire pressure, a wheel alignment, and a good working steering stabilizer helps but does not yet make the grade, then consider replacing your stock front and rear stabilizer bars with heavy duty versions. Also a set of heavy duty Bilstein shocks.
Our rig built on a 2007 E350 chassis had some handling issues so I gave it the full treatment when the rig was new, and we continue to benefit from that investment. It was money well spent.