Hello and welcome to JustAnswer!
What they have told you is correct. The rotor bolts to the hub from the back side on a dually truck.
Obviously the first step is to remove the wheels, caliper and bracket. The axle shaft is next removed by removing the bolts in the center of the hub and sliding the axle out. Then the hub nut can be removed, and the hub/rotor assembly can be removed. Flip it over and remove the nuts that hold the rotor to the hub.
When it's taken apart it's going to need an axle flange gasket and a hub seal, so it's something you'll want to have on hand before the work is started.