I also have a medium BGE and the last turkey I cooked was about 14# and fit fine. Not sure that an 18# would fit because it was full with the 14# turkey. I cooked mine at the felt line indirect about 325 degrees. Hope this helps you.
Are you cooking right on the grill? I first did a meatloaf in a loaf foil pan. The second time right on the grill. On the grill is by far the best. Good smoke ring all around. Also used a drip pan the second time after the mess from drippings.
I had been a gasser for a very long time. I never showed any pictures of any of my cooks. After SWMBO has been after me for about ten years I finally relented and we got an egg. I always said that I didn't want to mess with charcoal and gas was so much easier and quicker. Since getting the egg and tasting a huge difference I find myself showing people pictures of my cooks and telling them how great they taste. I don't know if I have helped sell any but I know that some of them are checking it out. SWMBO says this is a cult(lol) and in a good way she is right.