I booked via a company called HJ Travel. The always advertise in the back of the various mags. I've tried to google it, but the correct one didn't come up.
They were very good, and everything went as planned. I just had to book flights.
The company in the US is called Eaglerider (they may just be resposible for the bike rental).
The format was what they called Route 66 and the Wild West tour. Basically, Joshua Tree NAtional Park, Mojave Desert, Grand Canyon, Bryce Canyon, Monument Valley, Zion National Park and then Las Vagas.
The tour was guided by a 60 years old cool due, who looked like an Easy Rider reject. He was fun, and not above a bit of severe unkindness to the Hogs. (we had a bit of a racette on the ay out of Bryce, but had to give up after grounding the fopotboards to much). The luggage follows in a support vehicle.
I only did 8 days , but the tour carried in thro Death Valley, Yosemite, San Fran, and then the Pacific Coast Highway.
It wasn't cheap but v good.
HJ also offer trips down the Baja penninsula, BMW hire, Chile etc. Look then up.