Welcome to the forum.
It is optional but It really helps if you automatically add your motorhome discription to your posts.
You have about the longest wheel base and overall length of a gasoline chassis without having the stability of a second rear axle. The amount of motorhome behind your rear axle is putting you in the position of "the tail wagging the dog". When a semi-tractor trailer starts to pass you will receive a side air push on the back left side that will cause the front of your motorhome to go toward the centerline of the highway. As he moves up along side of you the wind push reverses with the back left side in a negative pressure and your front now has the air push moving your front to the right. The similar effect takes place when driving with the wind blowing sidways to your travel direction.
When I added the Davis TruTrac bar to the front axle of my 36' Ford Bounder it was an improvement. Several years later I added a Henderson? rear axle trac bar and noted the difference immeadiatey. My vote is for the front and rear axle track bars on your motorhome with the long rear overhang. The front axle track bar will also be a benefit in case of a front tire blowout.
As a side note, the sway bars make the greatest difference for sideway rocking which is not your described problem. The spring loaded tie rod stabilizer may give you some steering benefit.
My seat of my pants evaluation, Dwight