One thing you may be running up against is that some older trailers ran on single stage regulation from the propane cylinder. If you have a two stage regulator on your propane cylinder, you may be regulating the pressure too low. This is usually identifiable by a very low flame on the kitchen stove.
Also, you should realize that your rig is 39 years old and many of those years, especially the latter, the appliances have had no maintenance. I'd suggest to break down the furnace and clean everything. A wasp's nest/spider web in the intake/exhaust is all that's necessary to cause a failure.
I'd also suggest that you discard the brake wiring and install new wire, vacating any series daisy-chained layout and rewiring the brakes all with separate wires to a parallel configuration.
I've done a lot of these things when I rebuilt my rig and there are pictures and descriptions in the thread in my sig line---the Cowboy/Hilton.
Best of luck!
