The Ford E450 is notorious for bad caster alignment which makes driving any unit a challenge. Had my E450 realigned with the appropriate caster adjustment (look up other Ford E450 forums for the numbers) and a spring-load "steer-straight" modification (about $400 and two hours installation time) on the front wheel hubs.
Now, the unit (a Coach House 232XL ) is a joy to drive on the interstates and no more swaying! Also, driving for hours does not tire me out because the unit wants to go "straight" without any effects from x-wind or being swamped from passing large rigs!
I hope this helps with your problem!
Let us know!
