That era BIRD and Big boy were first generation and there was a situation that developed as the batteries aged or were neglected. I first ran into it on American coach products and then Monaco products. The key was the voltage required to get the BIRD to engage the Big boy solenoid so house and chassis batteries were charged. Here was the scenario: Chassis batteries were down to 11.9 to 12.0 volts from storage or age. If the coach was then plugged into shore power, the inverter/converter would start charging the house batteries. When the threshold voltage of 13.1 to 13.3 was reached, the B.I.R.D. would then tell the Big Boy relay to engage and also start to charge the chassis batteries. After a short time, the "combined" voltage of the chassis and house batteries dropped below the threshold voltage and the Big Boy dropped out. Then the charge voltage from the inverter/converter would eventually regain the threshold engagement voltage and the Big Boy would again engage. This clicking on and off of the Big boy really annoyed customers. The "fix" was to start the engine and fast Idle @ 1000 rpm for 15 to 20 minutes to raise the surface voltage in the chassis batteries. Eventually the combined voltage of the chassis and house batteries would satisfy the BIRD and keep the Big boy solenoid engaged.
Fred