At 20 degrees you can run it hooked to an outside faucet (assuming they work), so long as you keep water flowing all the time through it. But the water may be really cold which reduces the efficiency of the membrane (does not generally harm it, just doesn't work as well).
But as Mebbid mentioned you can easily hook it up to an inside faucet/sink as well. You can get them to tap into copper like a fridge, to screw into the actual fixture (e.g. where the screen is on it).
Another possibility is hook it to the bottom drain on your hot water heater. Three caveats: (1) run a few gallons out really fast and make sure it comes clean, some tanks will have a lot of sediment (it's not a bad idea to do this in any HWH once a year), (2) check the maximum temperature on your RODI
input water, most are in the 90-110 range, and compare to the temperature coming out of that faucet -- it will be a lot lower on most units than the real hot water taps, but might not be that cool, and (3) check it more than once as you use it, make sure it doesn't have a cycle where it gets too hot.
But a faucet hookup is awfully easy unless you have wacky fixtures.