DIY 3 way refrigerator venting (drawing)

I would also seriously consider going with a proper rooftop mounted vent. You do not want to choke things off with that small 3 1/4" x 10" goose neck; fan or no fan. Build an insulated chase wall, the full width of the fridge, up behind the food cabinet, and curve inwards towards the centre of the van, until you're far enough in to accommodate the vent. (this is exactly what I did, and it works fine). It's better to vent out the top, rather than the side, as a good stiff breeze could easily force air back down into the chimney. This is much less likely to happen if roof vented. (and no, installing a side wall goose neck with a back draft damper is not an option in this case. You need as little obstruction/restriction as possible)