I have replaced 3 thermostats on Smokin Tex and Cookshack smokers (the newest was made in 2010). They had no seal around the stem of the thermostat. I assumed Smokin-It was made the same.
I just checked my #3 and #1 and there is a rubber ring around the stem that "should" keep water from entering the thermostat. I would call the fixture water resistant, not water proof. So I would still try to keep as much water off the top of the smoker in the interest of safety. A hotel full sheet pan makes a good cover and does duty as a carry tray as well. Hope I didn't alarm you too much.
Dave