Opinions about vented/opening rear windows differ. Some like Sportsmobile recommend against them. Wind always eddies at the rear. Our ancient 1991 Ford Econoline has factory awning windows in the back doors. If left open while driving, exhaust gets sucked in like crazy. Dust would also get deposited in slider window tracks. All of that is certainly manageable, but we've opted to simply avoid it.
Ditto what MsNomer says about Motion Windows. The special aluminum frames on their tall sliders are indeed curved to fit the sides of the van. However, the upper and lower sections are both flat glass, which when butted together form an extremely obtuse angle. Get the picture? They also make a half-height version that is totally flat. But that works, since it's short/narrow enough not to matter. As you can see on their website, that they don't hug the outlines of factory openings. They are rectangular -- very RVish. CR Laurence awning windows are definitely more elegant looking, but we'll be going with Motion sliders anyway.
Awnings and sliders both have pros and cons. Awnings can be left open in the rain, but not when driving (fast) like sliders can. The van's sliding door has been known to hit an open awning window aft of it. Awning mechanisms are often less beefy than sliders. Finally, I imagine the flat glass in sliders will be cheaper to replace if broken. That was our thought process anyway. BTW, our rain workaround involves a MaxAir roof vent and rain shields for the two front doors.
