I have the same type of smoker, without the vents, and I do believe you can smoke A turkey to much. I cover my turkey after about an hour. I put it in A tin-foil dish and cover with foil
I drilled A hole in the top of mine, and put in A temp. gauge to get the excact tempurature. I like to know where I'm at degree-wise
As I understand, the water is for time- temp.- control (slow rise). I was told that water evaperates at 200* anyway? I've heard of people putting sand in the water bowl. I put carrots and potato's in my water bowl.
My guess would be to close the vents. I know it sounds like backward logic, but maybe to much outside air is coming in. As long as you don't have an air tight fit to your smoker, air still enters. I noticed that if I removed my lid on top, and opened up the side door, my coals would take off, but the internal temp in the smoker went down, once I closed it back up. Takes awhile to get it back up there to.
Smoking A turkey does take A long time, but it's the best I EVER had. My buddy used to do his turkey overnight.
Every thing is closed up on mine and I run about 200- 225*
Good luck, and keep us posted!