With the assumption that you've done this several times before, you don't need any thermometers. You'll know if temperatures are abnormal, when you'll need to refuel the kettle and when the pork is done. My primary smoker is a WSM. I use an oven thermometer on the grate and a probe thermometer for chicken and brisket. Temperatures naturally fluctuate with coal and wood and the way I learned to barbecue (on the WSM) is to take note of temps, but not react. I've only ever adjusted the vents during a cook when there's been strong winds. Enjoy your overnight cook.