If you go that route, please tell everyone on the forum where you are camping and also everyone in the CG to evacuate! There's a reason propane tank heaters are so expensive, even a 20# model costs about $375! Uncontrolled pressure rise is an bomb waiting to go off!
You're not having your tank freeze at all. What you are having is a pressure drop due to temperature conforming to the laws of thermodynamics.
You hit the first thing you can do inexpensively - keep them full! Next thing, keep them out of a draft. Many folks who camp in the cold replace their vinyl or plastic bottle covers with a nice cover fabricated out of at least 2" thick foam insulation because it also keeps the regulator warmer/shielded too. RV regulators aren't the best performing in the extreme cold either.
