Rear position if cooking inside
If you plan on cooking inside, I would definitely have a fan in the rear vent position. I do my BBQ'ing outside, but don't want to haul around an extra stove for general cooking outside. That's what the cooktop is for. I like a harty breakfast of hashbrowns, sausage and eggs, when I'm hiking hard. I put the fan on about a medium setting, open the side window flap above the stove a bit (at a minimum), open the side window behind the sofa, and cook away. All of the greasy smoke / steam exhausts almost straight up and out. My camper looks as clean as the day it was built.
BTW, if you are buying a new camper and are pinching pennys, just buy the vent options and then install your own Fantastic fans. You can have more than two for the price of one. Search this forum for Fantastic Vent or Fan, and you will find great advice on doing the retrofit. It is very simple. I purchased the Fantastic vent for about $140, and followed the advice of one member who suggested simply installing the "guts" of the fan in the stock vent frame. The stock vent frame is made by Fantastic Vent and is the exact same form factor as the one that comes with the Fantastic Fan. Doing the retrofit in this manner saves you from having to scrape the old caulk and re-apply new caulk. Really slick!