I've only done one turkey--a 10 lb. I spatchcocked it and injected and brined it overnight. Don't think it was a "natural" bird but after brining, injecting, and smoking it in my MES at lower temps, it was the juiciest, most tender bird I've ever eaten. Skin wasn't crispy but didn't matter to me--nobody here but the hound eats the skin anyway.
Never tried a 20lb'er. I did have success in doing a 12 to 14lber in my weber kettle once. Small amount of charcoal on each side with the turkey in the middle. Had to add charcoal a couple of times, but it was good. I used apple wood for smoke since I didn't want a real strong smoke flavor, and it's been my experience that birds absorb allot of smoke. Crispy skin and juicy meat - I hit it dead nuts on that one. I'm not sure how big your smoker is, but I'd first make sure a 20lb'er fit's. Good luck...smoked turkey tastes great and is an added treat to any thank-giving meal.
I typically smoke three 18-20 lb all natural turkeys around Thanksgiving; one the weekend before, one on Thanksgiving, and one the weekend after. With the turkey issues out here last year I only got one done. I smoke them at 300F plus so they get done about the same time as in the oven, just a little longer.
I definitely brine for 24-36 hours, preferring to add a little Prague Powder #2 to my brine. Not much, only a teaspoon. It gives the turkey a slight ham flavor that goes well with the smoke. I also cut the amount of recommended salt in the brine in half, especially if it is a pre-brined turkey. I also inject the turkey breast.
I use a turkey cannon for smoking turkey, putting a little cheap white wine in the cannon.
If a roasting pan will fit in your smoker, put the turkey in the roasting pan on the cannon or a rack, then fill the roaster with onions, potatoes, carrots, garlic, seasoning, and about a half bottle of cheap white wine. Smoke the turkey and veggies uncovered for two to three hours, then seal with HD aluminum foil until the bird is at the right temp. With my smoker I put the cannon on one level and use the veggie pan as a drip pan. Turns out YUMMY!