For better temperature control with the electric heater, stop at Lowes and pick up a baseboard thermostat. Wire the thermostat so it controls the electric heater. I use this setup with a fan forced heater which also helps.
However, the best method I have found is to use flannel sheets and a 20* sleeping bag. Then let the camper drop down to about 50* at night. With the lower inside temp, the heaters don't have to work nearly as hard thereby reducing temp fluctuation. Other advantage is this works without hookups too.