I haven't bothered before, just having hookup over the winter for the Batteries, but over the weekend decided to also plug in a little 450W oil rad I have, set to minimum and located in the underbed garage where the water containers and most pipework is, and left the mattress foot propped up to allow airflow from garage to living area.
Also decided to leave the water heater on its minimum setting to avoid any risk of water inside freezing as a precuation (the extra energy use is minimal - when I turn the heater on when the van was around 2C, the heater came on for just a minute)