Full size tractor unit empty fuel efficiency. Best one?

I saw a really interesting camper van for sale a few years ago, thankfully away up in Scotland or severe danger it would be here now.

It was on a Merc 814 chassis cab and obviously was based on a horse box design body, though as far as i could see it had been a camper from new.
What interested us is that the rear step frame had been used for carrying motorcycles as it was a race team wagon for most of its life, but that handy section, the boot basically, being strong enough to support horses was actually big enough to fit a Smart car in, and with a little alteration would i suspect have been able to get a little Suzuki 4x4 inside, making the most of getaway holidays.