The previous motorhome we bought wandered quite a bit when I bought it. Here was the sequence of events I used to solve the wandering problem.
1. Made sure tire pressure was correct for the motorhome weight. Going to the high side of tire pressure for me reduced tire squirm and wander.
2. Started replacing worn out suspension and steering components.
3. Checked alignment and had a bit more positive castor put in.
The greatly improved the handling of the coach which was a 1975 GMC motorhome.
My final solution was to buy a much heavier coach that did not wander. This really solved the issue.
But for sure replacing the connector pieces in the steering helped a lot. on the old GMC.