For what it is worth, below are relevant parts of the CA vehicle code. Not very clear, but it looks like housecars (motorhomes) 40ft and under are specifically exempted from the 26,000 weight.
12804.9.
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(3) Class C includes the following:
(A) A two-axle vehicle with a gross vehicle weight rating or gross vehicle weight of 26,000 pounds or less, including when the vehicle is towing a trailer or semitrailer with a gross vehicle weight rating or gross vehicle weight of 10,000 pounds or less.
(B) Notwithstanding subparagraph (A), a two-axle vehicle weighing 4,000 pounds or more unladen when towing a trailer coach not exceeding 9,000 pounds gross.
(C) A house car of 40 feet in length or less.
(D) A three-axle vehicle weighing 6,000 pounds gross or less.
(E) A house car of 40 feet in length or less or a vehicle towing another vehicle with a gross vehicle weight rating of 10,000 pounds or less, including when a tow dolly is used. A person driving a vehicle may not tow another vehicle in violation of Section 21715.
(F) (i) A two-axle vehicle weighing 4,000 pounds or more unladen when towing either a trailer coach or a fifth-wheel travel trailer not exceeding 10,000 pounds gross vehicle weight rating, when the towing of the trailer is not for compensation.
(ii) A two-axle vehicle weighing 4,000 pounds or more unladen when towing a fifth-wheel travel trailer exceeding 10,000 pounds, but not exceeding 15,000 pounds, gross vehicle weight rating, when the towing of the trailer is not for compensation, and if the person has passed a specialized written examination provided by the department relating to the knowledge of this code and other safety aspects governing the towing of recreational vehicles upon the highway.
21715.
(a) No passenger vehicle regardless of weight, or any other motor vehicle under 4,000 pounds unladen, shall draw or tow more than one vehicle in combination, except that an auxiliary dolly or tow dolly may be used with the towed vehicle.
(b) No motor vehicle under 4,000 pounds unladen shall tow any vehicle weighing 6,000 pounds or more gross.
(Amended by Stats. 1983, Ch. 708, Sec. 8.)