Make sure your tire pressure is proper for the load the tires carry.
If it still wanders after following the advise above, there are other relatively affordable remedies.
Being a 2003, you could benefit from any or all of the following.
- heavy duty front & rear stabilizer bars (the Ford stock front stabilizer bar is near useless)
- heavy duty shock absorbers
- rear trac bar (especially benefitial if towing)
- heavy duty steering stabilizer (ex: Safe-T-Plus, Steer Safe, other)
For our 24 foot rig built on a 2007 E350 chassis, I had all that done 800 miles after buying the rig brand new. It's handled perfectly since....just like an SUV.