I can understand your disappointment about the specifications, but on a motorhome heavy enough to have large wheels and big drum air brakes, I think I would rather have the drums than the disks. The disk brake systems in this size, especially trying to combine air brakes and ABS, have been too problematic for my tastes.
Even with boosted hydraulic brakes, as used on my C and on Ford's A gasser chassis, there are enough problems caused by "just sitting there" that I would lean toward drums when the wheels are big enough to contain them. On a C, they are not.