If you're asking for an opinion on whether you'd be OK with your current plans, I'd say "yes" go for it. My wife and I plan on doing the same as you though many times the temperature's a bit higher.
Assuming your holding tanks [are not] in a heated area and you think the temperature may drop into the 20's try dumping a couple of gallons of windshield washer fluid in each tank, should help ya out and it's cheap anti-freeze. If on the other hand your tanks are in a heated area don't worry about it, just have fun!
I'm sure others will chime in with good ideas. Hope this helps a little.