I have the water tank in my existing stick-built inside, and it takes way longer to freeze as long as the heat is on, and if I leave the interior access door open to it, I can go a ways below freezing with it. The trouble is though, is that the waste water freezes in the grey tank, so we have to do dishes and such in a wash pan and through it out in the bushes. For draining the fresh water in cold temps, the drain valve needs to be right at the tank, and not outside like mine is too, as the water in the valve freezes even if temps are nice inside.
That definitely looks neater Rob.