I've done the same thing at least twice on my 1995 Holiday Rambler. The first time was right after I bought it and was cleaning it up and checking everything out. It was drawing a lot of electricity, and I finally figured out the water heater was on (while it was empty). It was probably like that for a whole day. And recently it was turned on while empty for maybe a half hour before I realized it. After all that it still heats faster than my friends new Airstream.
I'm tempted to take the element out and find out what brand it is.