How do front brakes work on a 1985 Southwind motorhome on a GMC chassis?

The brakes are most likely hydraulic and use a pump to supply pressure assist to the master cylinder the same way a vacuum booster adds force to standard power brakes.

Except for the fact that a hydraulic system can apply more pressure to the brake disks or drums than a Vacuum booster the brakes work the same as any other braking system since the loop made up of the master cylinder and individual pistons that apply the force to the shoes on drum brakes and rotors on disk brakes are the same as power brakes.

Because of the added force applied to the brake parts in this system the drums and rotors are different and must be rated for hydraulic service. Visually they appear the same but standard types will wear out prematurely.

The majority of GMC/Chevy motorhomes of this period in the 24 to 35 ft length use the Chevy P30 truck chassis as the undercarriage. This is also the standard truck of choice used in most UPS delivery vans and it is easier to find information on this chassis and it's parts by searching for either the P30 or UPS truck in parts catalogs. This chassis available in several lengths uses one of the following engines, the gmc/chevy 350, 454, or the 6.2 diesel and all use a hydraulic brake system. The same pump is also used for the power steering system.