The tank heaters if you have them installed have a thermostat that controls the heating just because it was 40* outside it may be no where near that where your tanks are mounted if you have been using your coach heat. Some heating elements are designed to come on when the liquid in the bottom of the tank reaches 43* and off around 64*
Jake my sidekick (yellow Lab)
2006 F350 6.0
2016 Flagstaff 26 FKWS
AF&AM of Idaho
My unit is not like yours but my tank heater light comes on when I flip the switch regardless of the temp. My switch for the propane side of the water heater had a bulb but it didn't come on when the switch was flipped although the propane side worked. I looked behind the switch and found there was no ground wire for the bulb leg of the switch. I added one. Now I have a lighted propane switch.