Your tanks may have a combination of propane and butane. Propane is fine as it will "boil" at a very low temperature, but butane stops boiling at about 32 degrees F. In warm weather, using either fluid is fine and distributors mix the two. You may have burnt off all of the propane and all you have now in your tank is butane. The only solution now is to heat the tank up to about 40 degrees or better. Or, have a propane distributor drain and refill your tank.
You could have moisture in your tank and that could freeze your regulator. I don't know how to correct that problem except to empty the tank. A propane distributor may be able to do that for you.
Finally, don't insulate your tanks. They need to absorb heat from the air even at below freezing temperatures to boil the propane. If you're going to heat your tanks, then insulation may be needed to keep the heat in.
