That would be very unusual to have rear drum brakes. I thought only the Argosy and some early 28' Airstreams had rear drums, all the later and longer Airstreams had disc brakes on all axles with the exception of the tag axle.
You can get the Gear Vendor set up with or without the driveline brake so you need to know for sure.
My 280 with an Isuzu has a Gear Vendor with driveline brake and it's great. My 345 has a Cummins with an Allison so an entirely different beast.
"Socialist governments traditionally do make a financial mess. They always run out of other people's money. It's quite a characteristic of them."
~ Margaret Thatcher ~