Is charcoal or the smoker at fault for low temperature smoking?
It's the smoker. I had one of these, and it is extremely flawed in design. The pan that holds the charcoal does not allow for proper air flow to the fuel. Contrast this with a Weber grill, where you put your fuel on an elevated grate with plenty of air beneath it. This Char Broil instead just has a pan that you put on a shelf. Before long your coals are buried in ash, and barely smoldering. FWIW, if you were to attempt sealing the doors, it would only exacerbate your problem, as the issue is with not enough air, versus too much. I replaced my Char Broil with a Weber Smokey Mountain, but I had considered some hacks to make it better. Your best bet would be to ditch the pan-on-shelf system, and fashion your own charcoal grate that is elevated from the bottom of the smoker. You want something that will both allow air flow to the fuel and allow the ashes to fall away from the fuel. Then you would be able to properly manage temperatures with the air vents.