I think if you were to run your primpay vent line to the output of auxiliary tank, it would be drawn down as used by the pump. You would want a disconnect in case you wanted to drive off with the truck so a quick disconnect, but also a way to turn back on the primary tank vent with a valve.
I think that will work and can be done simply. I have an idea and can draw it up if you do not already have a picture in your mind how to do that. It os certainly worth a try and would cost <$20.00 in parts from Ace Hardware.
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