You have it under control. A low temp of 27F is not a big deal.
As you indicated, disconnect the fresh water hose and drain the tanks. While I normally keep them closed when not actively draining, in this situation on sewer, I leave them both open. This assumes a day or two not several weeks. Yeah, a #2 will stick to the bottom but if you return to normal usage once you start moving (keep valves closed until 3/4 full), it will break up given a bit of time.
We camp in those conditions pretty regularly and don't sweat it.
While we prefer it cooler at night, no harm in keeping it at 70F if that's what you like. You will use a bit more propane but nothing excessive. Keep in mind, typically if the low is 27F, the high is likely somewhere around 45-50F, so during the day you won't burn much.