More specifically, when it comes to cargo on highways, weight almost doesn't matter at all with respect to fuel economy. Adding weight to the bed has nearly ze

More specifically, when it comes to cargo on highways, weight almost doesn't matter at all with respect to fuel economy. Adding weight to the bed has nearly zero aerodynamic penalty and so doesn't change mileage much at all as long as the road is level and is not start and go traffic. Towing on a highway, is primarily an aerodynamic concern because most trailers have the aerodynamics of bricks and are (partially) outside the designed aerodynamic envelope of the rest of the vehicle. ICE vehicles take a noticeable hit on range when towing, even considering ICEs become more efficient when under moderate loads. There's no reason to hope that EVs won't suffer at least as bad, and 50% range loss is normal in an ICE when towing something large like a camper.