Had an uncle that had an 84 Chieftain with similar problem on his aux tank. After checking everything as you have, vent, crossover hose etc, we ended up dropping the tank, pulling the sender and the "nylon" type sock had a kink right as the fuel goes up towards engine. We figured it was sucking/kicking shut. We also found the rubber hose to the crossover valve was soft and may have been sucking flat because of the resistance in the tank. I cut the sock off the inlet, replaced the fuel line from the tank and put a good in line canister type fuel filter on. He never had the problem again.
2005 Allegro Bay 37DB