I purchased a convection oven 8 years ago. I have made my turkey using it every year, and we LOVE it! Would never roast a turkey any other way. The first year was very scarry! I called the butterball hotline, and they were not much help. I finally searched online, and got the best information from the "National Turkey Federation" which can be accesed online. This is what I have from them: Unstuffed Whole Turkey in convection oven, roast at 300F
14-18 pounds - 2 1/4 - 2 1/2 hours. 18-22 pounds - 2 1/2 - 3 1/4 hours. You might need to add 1/2 hour for stuffing. I never thought I would cook a 14 pound turkey in 2 1/2 hours, and it really did cook that fast. I took it out of the oven and left it tented on top of the stove for 2 hours because I gave it 4 hours to cook, never thinking that these directions were accurate. It was perfect.
As a side note, my father owned a large poulty coumpany that has since become a branch of Perdue poultry. My father would never sell or allow my mother to cook a turkey larger than 14 pounds. He said that Tom turkeys are tough, and should only be used for cut up dishes. Anyway, for years my mother and I have made 2 turkeys, anywhere from 10-14 pounds. When cooking, we put them in the same large roasting pan, side by side. Have one face frontwards, and the other in reverse. It still take the same cooking time as a 10 or 14 pounder. So much better.