With a pop up camper you already have a very low vertical center of gravity. There isn't much you would be able to do it change it other than packing a bunch of stuff on the floor of the camper which is probably overkill. Sway is more apparent on the newer / larger campers that have their floors above the rails. These campers aren't "top heavy" in the since that the top is heavier than the bottom, just that the bottom of the unit sits higher up on the truck overall.