The typical approach to ventilation is cutting a big hole in the roof. For those of you that want this, more power to you. I did not (and still do not) want this, so have had other ideas ever since beginning my conversion. If you want to argue that a roof vent is better, please start your own thread.
My intended goal with this thread is to share my experience so far with products and ideas - what's worked and what hasn't.
My concept for ventilation is to eventually have the ability to change the air flow intake/exhaust direction from the ceiling and the floor. This should help maximize heat retention in the winter and maximize heat expulsion in the summer. The system is still in it's infancy, but I've made some progress with the selection of components and control methods. I have a 4-1/8" hole cut (finally) very near to where many people seem to put their floor vent, not far behind the driver's seat and on the left side of the van. From there I intend to have ducting that extends up to the ceiling of the van where air will be sucked out in the summer and blown in (at a reduced rate) during the winter. I have yet to build the cabinetry and ducting to support this, but have purchased and experimented with the following products and how they can be used:
SEAFLO 12V 320 CFM 11A Air Flow Bilge Blowers for Boats Bilge Ventilation
D DOLITY Marine Boat 12v Quiet Electric in Line Bilge Blower High Air Flow - 4 inch 10.3cm Diameter
3.3V 5V 12V 19V 24V 5A Adjust DC-DC Buck Step Down Voltage Regulator Module 1Pcs
Initially I wanted to control the fans with a PWM switched MOSFET through software (Arduino-based), but quickly discovered that neither of the fans appreciated that approach. I tried varying the PWM frequency and duty cycle but in every iteration the noise was intolerable or the frequency not a good match for the motor's inductance. The step-down regulator boards I purchased weren't originally for this part of the project. They were intended for powering small electronics directly from 12V DC instead of their 120V AC "wall warts". But after I determined that PWM wasn't working, I looked at the actual current requirements for the fan I was experimenting with at the time (Dolity) and decided it was well within the limits and to try it out.
I was happy to discover that it ran the fan without introducing additional noise and control throughout the voltage range was continuous and fluid. I could go from moving a slight breeze to full on typhoon. Unfortunately the Dolity fans (both direct to DC and with voltage control) are VERY CHEAP and VERY NOISY. The best way I can describe the sound is something like "clattery" No big surprise given the price.
But the concept was sound, so I decided to try connecting the SeaFlo fan for some low range trials. The regulator board probably won't support full voltage/speed of the fan but that's a future experiment. I ran the fan just placed in my floor vent hole overnight at about 3V, which provided reasonable air movement and was very quiet. I was a little surprised that even down to the minimum voltage available from the regulator (~1.2V) the fan reliably turned slowly. In the morning I noticed that the main chip and capacitors on the board were slightly warm but nothing to be concerned about.
My next steps related to this will be to build the ducting and do some modifications to the regulator that will hopefully allow digital control through software. If I find that the regulators aren't adequate for full power to the SeaFlo fan I've thought of two potential solutions: 1) Find a supply that's properly rated, or 2) use a relay (or MOSFET) to switch between power from the regulator and direct full power when I want to use it for high volume air exchanges (like when cooking).
I'm pleased that this has turned out better than the direction it was going at the start. I hope this may give others some ideas for non-traditional ventilation options as well.