Mattress topper cannot be too thick, as you have very little room in the cab over on the '07 and earlier models. I finally looked at your pic and saw what model it was. I see yours is a '07 or earlier.......... '08 and later ('07 chassis) has a much taller cab over area so you will not have the room to add a thick topper and not feel like you are sleeping in a coffin.
Look at the shape of the cab over area in the pics, '07 on the right, later model on the left. (5 cyl has a single air vent in the hood, six cylinder has two air vents in the hood)
Charles
