If your rig is on an E450 chassis, then it does have a factory steering damper. If this is worn (and especially if it has lost some of its oil, and has an air bubble), replacing it with an equivalent is inexpensive and quite easy to do and will bring about a noticeable improvement. It's literally two bolts with nuts to remove or reinstall it, and all are easily accessible under the front bumper. The original one on my class C had gotten an air bubble, which meant that for small steering motions it was elastic like a spring, while for larger ones it acted like a damper; and that combination does quite odd things for the handling.
Do also make sure that you have the alignment set up properly, with a healthy amount of positive caster, and appropriate front tire pressures for the load on the wheels. Fixing those if they are not correct will help a lot with the steering feel and tracking.