What does your instruction manual say? That would be the best place to start. These have surely come back into use, although I am well aware that in large parts of the country, they never went "out". My mother had one (and that was a LONG time ago). My instant impression of the oven is that it doesn't have a lot of ability to retain heat (like a large oven) so things will take longer, as you discovered with your chicken. However, the turkey will fill the oven more, so the heat will be retained.
ALL that said, in recent years I am finding that turkeys cook faster than expected or than they did a number of years ago. I cook a 20+ # turkey every year and it is usually done in under 4 hours. An instant read thermometer is a staple in my kitchen and particularly on T'giving for the turkey thigh temp!
