If at all possible I would recommend installing a static RO and tank in a garage or shed and then transferring the water. It's true you can take steps to help prevent frost damage on ROs but inevitably the day will come when you have to shed out £250 for a new RO because of the cold. A mate just had his go, he'd been spending hundreds on resin before he realised, it was a van set up.
As for draining uphill, with an RO, not a prob.
Simon.