If you've got the budget, a hose out the window into a van system with built on filter. If its pumped it will only take around an hour. Or if you can make space, a pumped ro system in the flat, hose out the window to the van. Then you don't need to worry about frost damage.
Its only an hour. Or, cheaper, (you can get a slower filter) a tank by the door filled by a hose out the window (I live in a flat and do this). Transfer pump in the tank, takes about 5 mins to fill the van so you don't get in anyones way. There are a few ways. Or, a relatives house, and compensate them for the water used.
Easy.