1. The tailgate should be removed when hauling the camper. People have damaged the gate by putting too much weight on it or having road debris kick up.
2. You want the weight to sit as low as possible in the truck bed. If you need to raise the camper to clear the truck cab or bed rails, that is a necessary compromise to make it fit.
3. If you use foam to raise your camper, make sure it is a high density material. All the weight of the camper is concentrated on the edge framing and will compress the softer foam to the point it will bow and damage the camper underside where it does not compress.