Update: I ordered a new Trombetta solenoid, same part # as the original. After disconnecting the house and chassis batteries I replaced the old relay with the new one. Sure enough, after the engine runs 3 minutes the BIRD fires a signal to the relay, the solenoid clicks and you can hear the engine pick up the alternator load. There is now no voltage drop- it measures 14.0 on both posts and the engine alternator now charges both the chassis and house batteries. Success. The hard part was getting the old relay mount unscrewed.
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