COG

Anything you add to your truck in bed is going to change your COG. Unless you plan to make it yourself, COG is already factored in by the manufacturer of the popup.

I had a Sunlite folding TC unfortunately. It did have the upper storage.
Shocks that can handle the added weight

ajriding ~ thanks for input~~ concerning your last sentence ~~ have you ever been in a 4WheelCamper "Hawk' shell?? No upper cabinets. No cabinets attached to walls. Folded down exterior height(including vent)59" Deployed interior height 6'6".

Transvers bunk below cab/camper window ~ bench/wheel well high. Some clothing on lower bunk. Storage below. Heavy items on floor.

TakeCare ~ TravelSafe ~ HaveFun

Do a non-google internet search for the old style metronomes. They have a long needle with a weight, as the weight moves up the tempo slows.

Other than factory numbers I do not know how to calculate COG vertically. It would be the center of mass so best is to guess.

Like the metronome, if there is weight at the top, and this can even be your clothes bags and pots n pans in the upper cabinet, then you have a lot of mass at the top and when the vehicle sways it will sway slow. All vehicles will sway, even a formula-1 car, but the resonance (time it takes to sway right to left) is so much quicker on a car than a tall vehicle that it feels much more noticeable at the slower rate.
With the tall height the weight that is swaying is closer to enough-mass to overcome the springs and shocks it is leaning on so has to go farther into the suspension travel before it gets pushed back. The danger is going too far and then toppling the vehicle.

I had a class-C with worn shocks and the sway could get dangerous if not corrected immediately. I drove it so much that I was good at just counter-steering to counteract the sway and stop it the first swing, but when a friend drove it the sway got so bad I thought we were going to flip. I put everything on the floor, and that help tremendously.
I got the shocks and worn parts fixed quick after that.

Your fold-down will have storage under the top bed, and even clothes are heavy at that height. Most have cabinets attached to the ceiling. If you go off road and need to lower COG then your stuff will be better on the floor.

adamis ~ Thanks. That confirms my thoughts. Bottom of camper sits 1" off truck bed. Most of stuff in Boxes on floor(under lower bunk - side bench high -across bed)

TakeCare ~ TravelSafe ~ HaveFun

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.