[gregoryx]

I agree with all the comments, and would like to add. We have our galley up front, so when cooking, it is nice to draw out moisture or smells, a forward fan seems better suited for this.

The rear fan has the advantage of doing a full-van pull from the front windows; we crack them and/or install the metal louver vents when parked - depending on desired flow. When cooking (galley on passenger side, stove overlapping the slider opening), we put the fan in "push" and gap the back of the slider, thus venting directly above the stove. Just another way to do it.

Worth at least considering the split air-conditioning as well: more expensive, but more powerful and doesn't use roof-space. We have the evap / blower mid-ship blowing front/rear, which seems to work well.

If we'd done a roof-mount A/C, I'd lean toward the mid-ship mount (easy enough to shore-up the cross-members) to keep the bets of both.