A teaspoon of water inside a commercial freezer will freeze solid in a matter of minutes, but a few degrees below freezing for a few hours should not be a problem for your RV plumbing or tanks.
The rate of heat loss or transfer is a function of 1) the temperature difference between the two items, and 2) the difference in mass of the two items.
John & Diane, fulltiming since '12
'02 DS, FL, Cat, '04 Element
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element