Joe is right. But when you are confronted with a situation like this you really need to consider the risk / reward ratio.

For short periods of near feezing temperatures it is likely nothing will freeze if it is just an overnight low temperature. But each time you get away with taking a risk like that human nature will permit you to push the envelope a little more each time. To save a few bucks worth of diesel fuel so you do not have to run your generator or Webasto you could face some serious expenditures if things did start to freeze.

BTW, from reading horror stories the broken pipe or fitting from a freeze is usually a small part of the cost of letting the coach freeze. The most damage comes from having a small leak hidden from view that does a lot of water damage before you are aware of it.