we used to do the same with our ambulances. had to keep the IV fluids warm. the last thing you wanted was a giving a patient already in shock extremely cold fluid.Years ago I use to put a space heater in my truck. I would wake up and plug it in, by the time I was ready to leave the house (about an hour) the cab was warm and the windows were defrosted.
we just plugged into the shore power which you're already doing with your engine block heater. I think these other answers don't really understand the engine block heater. it's not producing enough heat to heat an interior if you were running the vehicle heater fan. it just keeps the block from freezing and they will getting too thick.