I had a similar problem with my 2005 Itasca Horizon with IFS suspension. I chased this problem for years. I took my coach to three truck tire dealers and two Freightliner service facilities for balancing, alignment, etc. Still had the shimmy. I had Equal installed in the tires. Still had the shimmy. I bought new tires and aluminum wheels. Still had the shimmy. On several occasions, Freightliner in Gaffney advised me to have the radial runout checked. Both the tire dealers and the local Freightliner service facility told me the front wheels were in spec. Now, I highly doubt it. They either didn't check it or didn't know how to properly check it.
Last April, I took my coach to Tolleson Freightliner, west of Phoenix, AZ. Prior to arriving, the service manager, Kirk Ault, told me he could fix the problem. When I arrived, they removed the Equal and balanced the tires on the coach. They found the Michelin tire/Accuride wheel combination needed no weights. They were in balance. Then they checked the radial runout and found my front wheels had .06 radial runout. The spec. is .04 or less. They dismounted the tires and rotated them on the wheels until they got around .035 on both wheels. This completely eliminated the steering wheel shimmy. Now, it drives very smooth with no vibration. My problem was corrected after driving 37,000 miles with the shimmy.
My suggestion: Find a dealer who will assure you they can measure the radial runout properly and will "work with" your front wheels until they are in spec.