Probably not a lot of demand in the US
There are firms that set up travel itineraries for road trippers, but I doubt there is much demand for a trip that lasts more than a month. One such firm, that supplies RV's is www.tracks-trails.com.
Actually, there are bus companies that could provide such programs. How many people do you want to travel with? I know of firms that cater to German tourists that use buses equipped with sleeping berths and those tours cover vast areas of the southwestern USA.
TrekAmerica specializes in 3-week trips (for the 18 to 38 year old traveler) and uses 12-person vans.
M.