I suspect the main culprit is Ethanol, that's why I asked what kind of gas you guys were using. The water separation problem (aka phase separation) is made worse in cold weather, which turns the water gel into a thicker, sludgy gel. So, the more Ethanol in the fuel, the longer the fuel sits in the tank, and the colder the weather, the worse the sludge problems will be.
The Ethanol conditioner additive seems to be a good solution. At (almost) every fill up, I add an ounce of this: