CamperBerk, The low speed shimmy (30mph) is a good indicator of a tire problem...excessive runout caused by a bad belt, high point (run out) of the tire matched to the high point of the rim or it could be a defect in the tire. I'd take it to a tire shop and have them do a runout check of the tires. Normally, an out of balance tire will not cause a shimmy/vibration at such a low rpm=mph.
As mentioned, rotating is a good trouble shooting technique and may quieten down the shimmy/vibration, but you'll still have an out of round tire on your RV. If your tires are still relatively new, have the manufacturer's dealer take a look see...may be covered under manufacturer's warranty and eligible for an adjustment. Good luck, Bob