Wouldn't it be simpler to just get the energy directly from the alternator or if you are ambitious a second alternator? The source of any energy generated is the diesel you are burning. I'd be surprised if the efficiency of a wind turbine mounted on a van as expressed as energy out versus extra fuel burned is better than 5-10%. Hybrid vehicles retrieve extra efficiency from the car's kinetic energy because the extra resistance can be turned off during times when slow down is not desired. Even if you could selectively let a wind turbine free spin when you don't want resistance, the blades and the structure that holds the turbine will have significant effect on MPG. It sounds like a fun project for a tinkerer to play with but from an efficient energy generation point of view it's whack-a-doodle (no offense).
Yeah totally a paper exercise, no offense taken at all...in practice I opt for a small 600W aux inverter while vehicle is running as backup to solar, which is still inefficient but less hassle running small AC wires to the house inverter/battery bank, and using common low-cost components rather than installing aux alternator with specialized DC-DC charger with long thick 12V cables. For the odd times I need it, I'm ok with the tradeoff.
Back to wind, the thought is it would be generating while vehicle is moving. Desired for lower profile likely means insufficient output and/or not much output from natural wind while stationary. From fuel to electric output point of view, it is going to be poor...but you are driving to some where anyway, and the slight drag increase would not be much more than say the roof top AC, the large side mirrors, or various accessories we have on the van.
IMO the most challenging issue is package size. Given what I see online of pole-mount home turbines making 100-150W on a good day, scaling down for vehicle applications would be the physical limitation...money, research and development may solve that problem?