I am wondering if you may be encountering a problem with the Airstream wiring color/function of wires compared to the modern industry standard. In 1966
, Airstream used its own wiring pattern that is not the same as today's industry standard. You can find a pdf file linked below that will identify how your coach was originally wired. You can compare this data to the Marks RV
data to get an idea of some of the possible issues.
There is also a possibility that a wire may be loose or disconnected in the umbilical cord junction box. The junction box is usually found in the front curbside corner of the bellypan -- it can be identified as a recctangular "patch" on the underbelly skin.
If someone has at some point pulled out the breakaway switch and didn't get the pin back into it quickly enough, your coach's brake magnets may have burned out. Another possibility is that a blow-out on one of the wheels may have sent tire rubber into the brake wires severing one or more of them.
Good luck with your investigation!