Filling water tank in cold weather

I don't think you'll have a problem either especially once the water begins to move around when you're underway. If it were me, I might run my furnace for an hour or so late Friday. If you do that, it's doubtful that your actual water temp would even approach freezing at those temps, let alone freezing solid, which is what you want to avoid. As mentioned and you've indicated, don't run the pump.