Do you have an E450 chassis or an E350 (or something else)?
I certainly agree with the recommendations of making sure the alignment is correct and the tires are properly inflated. It would also be well to verify that the tie rod ends and ball joints are tight, which I think they ought to be checking when doing the alignment.
I think the E450s all come with a steering damper from Ford: basically a horizontally mounted shock absorber for the tie rod. If it's worn, replacing it with an equivalent or OEM part (not necessarily a full-fledged steering stabilizer with spring centering) can make a noticeable difference. It's an easy part to replace: readily accessible, with just a couple of bolts to hold it in.
On my motorhome, when I bought it from the previous owner, the damper was definitely shot and had leaked some of the oil charge out over the years. When I replaced it, I discovered that the old one would act like a spring for small displacements (due to the air bubble compressing) and like a damper for larger displacements (with the remaining oil circulating as designed). Needless to say, that caused a bit of odd steering feel.
One other suggestion: when driving, keep your focus as far down the road as practical. It's easy to overcorrect when driving a motorhome or other long-wheelbase vehicle if you aren't particularly used to them. Steering corrections take longer to have an effect, and the result--doubly so if you're focusing on the spot right in front of you--is a tendency to overdo them and end up overshooting towards the opposite direction. Making small adjustments and looking far down the road are the solution. (Looking farther down the road is likewise advisable to give yourself plenty of time and room to brake when that's needed.)