I went out this morning to check after I read your post. Not a very cold start to the day but, using a Kill A Watt unit, I see a pretty consistent 7.75 amp draw when I plug the block heater in. My 6.0 used to draw @11 amps to get up to temp. I use a timer during the winter which turns the heater on 2 hours before I leave for work so it doesn't run all night long - not sure if it cycles 'on-off' when it finally warms up.
You will occasionally find threads in the RV forums where somebody plugged their unit in during the late fall before winterizing and storage and then forgot to unplug it. When springtime rolls around and they go to retrieve the RV from the storage facility they are presented with an $800 electric bill....