Put a cup of water in your freezer, and see how long it takes. Then imagine how much bigger your water tanks are, and how much warmer it is outside than your freezer.
If you are really worried about water lines, turn the faucet on so there is a small trickle overnight (provided you are hooked up to campground water!). Ditto for your gray waste line.
I wouldn't at the temps you are talking about, but it will give you added protection if you're feeling unsure.
It's been High's of 35-37 (F) and lows of 24-28 the last several days here in Minn., and my outside garden hose isn't frozen fully yet (and we had some serious cold a while ago, that should have gotten it well on the way to freezing!)
2012 Ford F-150 Eco, Max/Max
Jayco 26.5 RLS, Reese Sidewinder