Fresh Water Tank Freezing

I live in area where it occasionally gets cold enough for freezing damage to be a legit issue - so I winterize the rig every year - which includes dumping the FW tank. But I would really like to leave the FW tank full of water until it gets cold enough/long enough to make it a legit issue. Rig is used as "life boat" during winter during power outages etc - having convenient water nice.

Anyone give me a guesstimate on how long it would take to freeze 30 gallons of water? I know the starting temp of liquid is relevant so maybe we can assume the rig/water container are all 32 degrees as a starting point. I also know that keeping the rig heated would make a difference but for now lets assume it's not.

Comments appreciated.

Thanks!