I think I have the exact model you have. I just got it a week or so ago from HD. Has 2 doors in the front and a large square charcoal pan with a little coal grate in the bottom?
I haven't had much problem with my temps, except when I stop paying attention...
First, do trhe3 coalp[an mod. Drill a bunch of holes along the sides of the coal pan to allow airflow to hit your coal. Also, drill a bunch of largish (1/2 to 3/4 inch) holes in the bottom of the pan to allow ash to fall bown. I placed a foil lasagna pan in the bottom of my smoker to catch this ash. When I notice the temp dropping a b it, open the bottom door of the smoker and shake the coal pan, knocks off the ash and allows a hotter coal burn to keep on keeping on.
I've been told to keep the top vents open, but find that keeping one nearly closed and one open about half way works for me,. But the wind airflow in your location will be different than that on the side of my house (unless you want to brink yours here --Virginia Beach-- but that might be inconvenient!). I start with the bottom vents wide open until the coals get burning really well (lit outside the smoker and placed in once they start ashing pretty good). Once my temp gets up a bit, like 220, I close them down. One all the way and one just cracked a little. Food goes in and the temp drops so open one or both back up to get your heat going then close them up again, Then it's a matter of adjusting the bottom vents to raise or lower temps, and shaking the coal pan to knock down the ash, of course as needed. I find I have to add coal/chunk wood every 1-1/2 to 2 hours.
Keep the top door closed as much as possible. It drops your heat a ton and you lose all that beautiful smoke!
I also run a digital thermometer through a top vent and stick it through a potato so the end doesn't touch anything to watch my internal temp. The one on my door is off by about 60 degrees. Ignore it!
OH, when you add water, use the hottest you can get into it. Some people suggest using sand in that pan instead of water...,
Hope this helps!