Remember, when you install any type of this type ventilation, YOU MUST have a way to input fresh air. Houses have vents on the underside of the eaves to allow fresh air to enter and then exit. You will have to figure out a way to allow fresh air to get into the RV before the roof top vent will work. There is NO attic in a RV. There may be a space depending on the design of the roof itself. Jayco's have Styrofoam insulation in the roof and NO free space. I cannot think of any RV that will have any type of free space due to the lack of thickness in the roof. ANY space will be filled with Pink insulation or Styrofoam. If there IS space, then you will have NO insulation keeping the sun's heat from going right thru. 5th wheels are not built any different from all other RV's. Doug