My hard side has an AC and it is 12'6" above the ground. Bridges or overpasses that are lower than 13'6" few if any exist on the highways. Check the states websites as they provide information on low clearance areas on highways for the trucks so they don't get stuck and shut down the road.
The hard sides will not affect your fuel economy if you stay at 60 MPH or slower when traveling. At 65 MPH or higher speeds the fuel economy will be hurt as the air drag increases with the square of your speed. This includes any head winds as well.
I would only go with a popup to have the lower center of gravity for serious trail running that is truly off road. Otherwise the drawbacks so well covered by the OP are what led me to go with hard side campers.
The Northstar Liberty will provide for AC, easier access for quick stops, and a more storage space. We use the cabover space of the hard side camper for all our personal gear so it is easy to get to and this includes clothes in duffel bags, photo gear, laptops, etc. and this provides more room in the cabinets for other stuff. At night most of this is put on the dinette so the bed is clear.
