It seems a bit excessive to me. I think that P30 chassis is just a 1 ton dually with 12500 GVWR. I know they used to put way bigger RV's on them than they should have and they always sagged in the rear after a couple years. Like others have said they seemed to mark the prices of parts up 2x retail. Labor might be OK, I think they have drum E brakes and might have to pull the axle to get new pads on Hence the rear end service.
Pretty sure those had the e-brake on the back of the transmission.