Henry,
If you plan to overhaul the brakes as OEM, I will make an expensive suggestion. That may be the least expensive system correction.
Call Applied (JimK, well Nick actually) and ask about the Drum/Drum arm set.
This only requires one small modification of the OE parts, and then all four rear brakes work as they should.
With a coach as light as a Birch, you may be able to lock one of the four rear tires, but it will not be the same one twice in my experience.
This made a remarkable improvement to my brakes and the same is reported by any others that I know of.
Matt