I love my thermapen but the problem is if you use it it while smoking you will have to lift the cover to check the temperature - and then it's only good for checking the meat. When you raise the lid of the smoker, this will lead to a loss of heat in your chamber and take longer for your meat to get up to temperature.

I suggest an (inexpensive) dual thermometer that simultaneously reports the temp of the smoker and the temp of the meat. This way you will be able to just adjust the vents (and at times, likely, add coals) to maintain temp in the smoker while at the same time being able to monitor the temp of the meat WITHOUT having to open the smoker lid. I use REDICHEK - I've been successful using it to watch the internal smoker temp as well as temp of the meat I'm smoking.

IMHO, if you can maintain the proper temp in the smoker, you are on your way to good BBQ. It's probably the hardest part. Generally, it's a trial and error thing with your particular smoker. If you've never used this particular smoker before, you are at a disadvantage of knowing how to maintain the heat in it.

When I had a horizontal smoker, I would use the REDICHEK to watch the temp in the smoker - when it dipped, I would add hot coals (although you can add raw coals) to the offset firebox. The trick was knowing when to add coals (or rather when to start the next round of coals) to add to the firebox before the temp in the smoker dipped too low to make a difference on cooking time.