Autos with trailers and trucks (vehicles subject to the 55mph limit where other vehicles have a 65 or 70mph limit) are not generally restricted to the right lane in California.
They only are restricted to the rightmost lane on roads where: (1) where there is no other designated (by sign) set of lanes for such vehicles, and (2) The road is not a divided highway with four or more lanes in the direction of travel (where the default, without a designation, is that they are restricted to the two rightmost lanes for normal traffic, plus the next lane out for passing.)