The back door would be the ideal location for vents and an AC unit, but unfortunately is not an option for those of us with ramp doors that are building toy haulers (or in my case my race car hauler) where we need to use the ramp. If I could use that wall, I'd have a tractor-trailer reefer unit hanging off the back of my trailer right now and be able to hang meat inside all summer!
When I bought the trailer, I originally picked up two of these style vents intending to put them on the side walls:
http://www.easternmarine.com/Aluminum-2-Way-Pop-Up-Roof-Vent-7-3/4-x-13-3/8-10-091-0/
When I held them up against the side of the trailer where I wanted to mount them, they just didn't look right and stuck out too far.
I am still thinking about the half moon vents:
http://cgi.ebay.com/ebaymotors/Trailer-Side-Wall-Air-Vent-1-2-Half-Moon-NEW-BLACK-_W0QQcmdZViewItemQQitemZ200466475746QQptZMotorsQ5fRVQ5fTrailerQ5fCamperQ5fPartsQ5fAccessories
The third option is sliding windows. In my case, whatever option that I choose must have fine mesh insect screens, since I live surrounded by woods/wetlands and there are days when I have to put on jungle-strength Deet just to walk out to the mailbox.
A mechanic friend and I have been toying with the idea of mounting a horizontal-shaft lawnmower engine and a car AC compressor on the tongue of the trailer to make a gasoline-powered aircon unit, with the refrigerant lines routed through the floor.
I almost wish that the factory had installed either roof or wall vents, so that I didn't have to make this decision because of all the available venting options. I'll probably wind up doing with the half moons for venting during storage, and windows for cross flow with the doors open when I'm using the trailer. I'd love a roof-mounted aircon unit as well, but having to cut a hole and brace the roof for it makes me hesitate to pull that trigger.
In any case, I hope this info is useful to the original poster, if for nothing else than to explain the available options.....