You can't argue that adding more weight or making the less aerodynamic doesn't require more energy (fuel) to travel.
However, the amount of additional fuel may not be significant.
For my rig, I get about 400km to a tank of fuel towing at 65mph on the highway.
Trailer mostly empty, truck mostly empty, no water = 400km
Trailer packed for dry camping, full water, 2 Generators, 2 extra gas cans, large tool box, = 400km
Full water, canoe on the truck = 400km
The 4.7L Toyota doesn't seem to care what I bring or how I load, as soon as I hook up the Airstream it's 400km till empty.
When pulling the 1500
pound, 13 foot Fiberglass Trillium trailer we get about 450km to a tank. Funny how pulling a trailer twice as long and 2 1/2 times as heavy has so little effect on fuel mileage.
FYI, the truck gets 550km to a tank empty.