I just went out and measured my 2008 Lance 1055. At the head where the nose curve stops and the flat roof starts you have 39" between the cab over floor and ceiling and you have 42" at the foot end (capet end to ceiling). Mattress thickness will determine your actual headroom.
2011 F350 6.7L Diesel 4x4 CrewCab longbed Dually, 2019 Lance 1062, Torqlift Talons, Fast Guns, upper and lower Stable Loads, Super Hitch, 48" Super Truss, Airlift loadlifter 5000 extreme airbags