I'm not sure why anyone would not expect squat when you load a heavy camper. Springs deflect under load, that is their function and what they were designed to do. Most pickups in the last decade or two will squat 5-6 inches between empty and full load. If you make the springs so stiff that this is down to 2", the ride when empty will be horrible, and full not much better. If you expect the headlights to be aimed the same empty and full, you need to change something in the springs when you load the camper, or aim the headlights each time.
Passenger cars squat also, but since the load is more equally distributed between front and rear, they squat close to the same and the headlight aim is not affected (as much).