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
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.