Help with auxiliary fuel tank decision or replacement.

You got it backwards. Condensation is pretty much a myth with plastic tanks, but is a concern with a metallic slip tank. Besides, just run em down low and fill when running the truck again.
I've never subscribed to the full tank for storage theory and I shelf many engines every year off season. Fresh fuel (especially if talking gas, not so much for diesel) trumps the potential of moisture IMO, in most cases.