With 'normal' vehicles tests showed that the best average fuel economy was at about 50 MPH or 80Kmh - as the speed increases the wind resistance increases as the square ... not a linear increase ...
the extra resistance with a towed van or the higher one piece campervan things will get far more expensive very quickly.

Also, as the speed increases, 'aerodynamic lift' of the body shape can cause the front steering wheels to lose good contact, thus making life exciting...