Does your water heater have a rocker switch at the water heater? If so, start making sure it's flipped off too whenever you drain the water heater. And make sure the switch is off inside the camper too.
If the water heater was warm, are you sure you drained the water? It's possible there was still water in the tank and the heating element was still under water. I think you dodged the bullet, but you really won't know for sure until spring.
I put a secondary switch on my on-board water pump too. The switches for lights, slides, water heater, and water pump are all in the same spot. Over the last couple years I have found the water pump running when the fresh water tank was empty. I don't know how that happened, but it did (several times).
I ran a second automotive type toggle switch, so both switches have to be "on" for the pump to run now. When I drain the water tank I always flip the switch at that time. Now I flip them both. The second switch is NOT any near close to the factory installed switch. It's actually on the wall under our bed. Since I did this, the pump has never been left running with empty tanks again.

