I believe my manual tells me to turn them on (switch is on control panel) if you expect temps below 40 degrees. They then have a thermostat that will turn the heat on and off as needed to keep them from freezing up. You could probably leave them on if you wanted, but the manual didn't say anything about doing that. Probably better to activate the system only when you know it might be needed than to accidentally risk heating an empty tank and melting something.
D_B Travelers - 30 nights in 2016... so far. 54 nights in 2015. 2014 Rockwood Windjammer 3001W Diamond Pkg, 2015 GMC 3500HD CC DRW Duramax, Equalizer 14K hitch, TST 507rv TPMS.
"Happiness? A good meal, a good cigar and a good woman - or a bad woman; it depends on how much happiness you can handle."
George Burns (1896 – 1996)