If done right, I don't think they will produce that much more drag especially on an SUV body (which aren't renown for their aero anyways). But as to the gains... not sure. I would use a solar rig set up to send power into a deep cycle battery and from that run my appliances (ie a small fan)
Something like this...
"How to make a solar power generator"
But I'm all about temporary gadgets that can be deployed and withdrawn at will. I'm not saavy enough to wire it into the car itself.