As suggested before, check for any excessive movement of the tow bar in the receiver. If you have any, install an immobilizer. The best built (IMO) that I found was the BlueOx:
Blue Ox Hitch Receiver Immobilizer II - 2" Hitches Blue Ox Hitch Accessories BX88224
I check the torque on the immobilizer once a year...we have NO movement whatsoever at the receiver after 4 years.
With the shorter gas chassis, the 'tail wagging' seems to be an inherent quality and can be pronounced while towing due to the lateral forces the towed puts on the rear of the RV. This is a known condition and a rear track bar should be a standard equipment item(AFAIC). The long arm of the tow bar and the distance from the receiver to the rear axle can exaggerate these latter forces. The rear trac bar is going to be your 'best bang for the buck' item that addresses the issue.
We installed the BlueOx Tiger Trac and found that it did so well that we could forgo all other fixes. BlueOx is very proud of it, but I thought it a well made, heavy duty unit...still doing it's job and works quite well. No affiliation.
Blue Ox - TigerTrak
Also, tow bar height at the MH should always be 1 to 4 inches higher than the height at the towed...never the other way around. This should be helpful:
Tow Bar Angle and Height Difference