Cooking Turkey in a Convection Oven

So I should cook them for 2 1/2 to 3 hours at 300 degrees F?

Using a convection over requires lower temperatures than conventional ovens; set the oven temperature for 300 degrees F. You don't have to baste the turkey during cooking, but if you're using a marinade or sauce, wait until the end of cooking to baste.
Cooking time varies by weight, but an unstuffed bird of 14 to 18 lb. should cook for about 2 ¼ to 2 ½ hours. If the unstuffed turkey is 18 to 22 lb., extend the cooking time to 2 ½ to 3 hours. Stuffed birds take a little longer; allow for a cooking time of 2 ½ to 3 hours for a 14 to 18 lb. bird, or 3 to 3 ½ hours for a stuffed, 18 to 22 lb. bird. If you need to open the oven door often during cooking, the time required will be increased due to heat loss.

There are a few other variables to keep in mind that may affect cooking times. If your turkey is partially frozen, cooking time will be extended and the turkey may over-brown. To slow browning, cover the turkey with a foil tent; this will extend cooking time as well. Shinier metal pans cook turkey slower than darker roasting pans, and the larger and deeper the roasting pan, the greater the cooking time, due to reduced heat circulation around the turkey. Finally, if you use a lid on your roasting pan, the cooking time will be reduced. Good luck.