The industry standard for RV roof vents is 14“x14”1) and can be as simple/inexpensive or fancy/expensive as one wants.
See also non-standard vents below.
There several and sometimes conflicting factors affecting vent placement.
Some vans have flat (or flat-adjacent) areas to facilitate mounting fans.
If no entirely flat spot is available, one can cut through the corrugations and either use an adapter to seal the gaps or build up the low areas with buytl tape, etc.
OEM vents are very simple and will let rain in if left open. There are several ways to deal with this issue:
Be diligent about closing the vents manually. Easier said than done.
Install a vent cover; this will allow for ventilation during bad weather. It also has a bonus function of decreasing sun and weather damage to the actual vent. Be sure your chosen vent is has enough room for complete movement of the vent's lid. Application notes:
Select a vent with a rain-sensing function that will close the vent automatically, like the Fantastic Vent 807351. The manufacturer explains: “When dome (lid) is open and moisture contacts the sensor, the dome closes and turns fan blade motor off if it is running. When the rain sensor dries, dome reopens and fan blade motor runs if fan was running when dome closed.”10)
More expensive fans have features that some will find worthwhile:
Since hot air rises, you may get best results in summer by using the fan to blow air out of the camper. If you have a ventilation intake under your vehicle you may be able to pull in relatively cool air.
In winter you may want to run the vent on the lowest setting pulling air into the camper. This will preserve your warm air at the roof, contribute to circulation of air, and help control humidity.
Fans will use various levels of power at their different speed settings. For example, a 10-speed Maxxair consumes between 0.1A at lowest speed to almost 3A at full speed. See the pic to the right for one user's measurements.
The base 3-speed Fantastic model reportedly consumes 1A, 2A, and 3A respectively.13)
Although 14×14“ vents are standard in the camper world, there are other (usually smaller) options. The smaller vents often have greatly-reduced flow, as measured in CFM14). Some of the smaller ones give cubic feet measurements, but are given for an hour. Example: 900 cubic ft/hour == only 15 CFM.
'Dwellers looking to maximize solar real estate may be able to mount low-rise vents under raised solar panels.
If you have a van with solid rear doors, or a van with a lot of space above the door windows, there are wall-mounted vent fans as well. https://nomadicsupply.com/product/pika-van-outfitters-rvf-rear-vent-fan-system/