Just an idea, and it's cheap to try. I use the long stem dial temp that came with my turkey fryer. I drilled a hole in the side of the drum just big enough for the stem to go thru. The tip of the stem ends up close to the center inside of the drum. Its a couple of inches below my lower cooking grate.
I also have a dial installed in my lid, and it usually reports 30-50 degrees cooler than the turkey fryer one.
My preference is to know approximately what the temp is right under the meat, rather than out by the side of the drum. Just my .02.
I think the temps vary quite a bit depending on where you measure them in the drum, and I completely agree with what cowgirl says.