When calculating energy requirements on a flat plane of travel, the weight of the load is only relevant* when accelerating. On a flat plane, once the load is up to speed, it doesn’t take any significant increase of energy to maintain speed, regardless of load weight. The main reason fuel is required to maintain speed on a flat surface is overcoming air resistance. At the speeds that road traffic travels, only the front and back shapes of the vehicle contributes substantially to air resistance, not “air friction” from an elongated middle body. This means that on a flat plane like a highway, doubling the length of a vehicle to accommodate double the load, does not require anywhere near double the energy to maintain speed. It does however provide much more space to capture solar energy.

By the way, you can take advantage of this yourself on long highways by driving close (but safe) to large trucks, driving in their slip stream. It cuts fuel requirements substantially because the truck is doing some of the work of moving the air for you.

One puzzling thing is why trucks aren’t designed to be more aerodynamic, instead of a giant box shape. Anyone got any thoughts on this?

*aside from small things like added friction on bearings, changed tire geometry etc.