Unfortunately the local DMV and whomever they called in Sacramento gave you the wrong information. I know it is a moot point for you since you are no longer licensed in CA, but I want the correct information to be given here. The applicable CA vehicle code section is 12804.9. It is a long section that describes all the classes of licenses in CA. The 5th wheel endorsement for the Class C license is very specific as to weights in this section. Here is an excerpt specifically pertaining to Class C Licenses (I bolded, italicized and underlined the applicable sub-section for this discussion):
(b)(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.
The authority to operate combinations of vehicles under this subparagraph may be granted by endorsement on a class C license upon completion of that written examination.
(G) A vehicle or combination of vehicles with a gross combination weight rating or a gross vehicle weight rating, as those terms are defined in subdivisions (j) and (k), respectively, of Section 15210, of 26,000 pounds or less, if all of the following conditions are met:
(i) Is operated by a farmer, an employee of a farmer, or an instructor credentialed in agriculture as part of an instructional program in agriculture at the high school, community college, or university level.
(ii) Is used exclusively in the conduct of agricultural operations.
(iii) Is not used in the capacity of a for-hire carrier or for compensation.
(H) Firefighting equipment, provided that the equipment is operated by a person who holds a firefighter endorsement pursuant to Section 12804.11.
(I) A motorized scooter.
(J) A bus with a gross vehicle weight rating or gross vehicle weight of 26,000 pounds or less, except a trailer bus.
(K) Class C does not include a two-wheel motorcycle or a two-wheel motor-driven cycle.
If you are a licensed CA driver, you can tow a 5th wheel that has a GVWR of 10000 lbs or less on a Class C license with no endorsement. If you tow a 5th wheel with a GVWR between 10000 lbs and 15000 lbs, you need the above described endorsement on your Class C license. This endorsement is obtained through a simple written test. If you tow a 5th wheel with a GVWR greater than 15000 lbs, then you are outside the the Class C license restrictions and fall under a Class A license in CA. As long as you are towing the 5th wheel for recreational purposes only, then it would require a Class A non-commercial license. The testing for a Class A non-commercial license is basically the same as testing for a Class A commercial license in CA. The difference between the two licenses is the medical certificate required. The non-commercial medical certificate is a form you fill out self certifying your medical ability to obtain the license. The commercial medical certificate requires a properly licenses medical practitioner to examine you and certify you qualify.
Most DMV and LEO personnel don't know this information (commonly) unless they have been educated on it specifically. I am a LEO in CA and have been educated on it specifically through trainings related to vehicle code enforcement. Above is the actual vehicle code to back that up. I do my best to educate others about this because I know it is not commonly known by even those responsible for issuing licenses or enforcing driving privileges. I know of many people who have gone into the DMV to attempt to get the 5th wheel endorsement on their Class C license and have been told there is no such thing or have been given incorrect information as Twotoes was. It is unfortunate, but it happens. The CA vehicle code is a massive document with many obscure sections that no one could know completely.