In highway driving, weight of trailer isn't as much of an issue as how much of a barn-door it is in terms of aerodynamics. We've noticed when towing enclosed trailers in the 5-by-10-foot size range that they have less of a fuel consumption impact behind my van than they do behind a pickup truck ... they mostly fit within the frontal area of the van and the long roof of the van extending all the way to the back should lead to less flow disruption when it in turn hits the top of the trailer (which is same-height or perhaps below the top of the roof of the van).
It's very easy with pickup trucks with gasoline engines to send consumption into the 24 - 25 litre per 100 km range by towing a big-brick-shaped enclosed trailer.
It's very easy with pickup trucks with gasoline engines to send consumption into the 24 - 25 litre per 100 km range by towing a big-brick-shaped enclosed trailer.