I'm not sure how much hood length you guys will need because I don't know your equipment list nor what your cooking. But I'd suggest again that you check out Ebay and take a minute and call Ventilation Dirrect. They have items on Ebay but you can always get a much better price if you call them on the phone. I have posted here on RF about them and my unit came from them.
I know you guys are thinking a 12 volt system and I'm not sure why but even if you don't run a charbroiler or flattop grill a really good exhaust system is worth it's weight in gold. My exhaust fan runs from early spring to late fall just to cool the trailer. It drops the interior temp a bunch pulling air through two screened windows and out the roof.
There are a bunch of exhaust hoods on ebay and you also need to search exhaust vent.
Here is an example http://cgi.ebay.com/NEW-5...on?hash=item5ad6a7f2ab
And I have bought some used equipment from this company, but I'd just about bet (both companies are close to each other) they are dealing with Ventilation Direct and just having them drop ship what ever you buy.
A 12 volt system may get you past your local HD but are you going to be able to work when it's really hot outside in midsummer and you have a bunch of heat producing equipment running?
And I understand (short on money) but there are places and times to cut and some times you just have to spend the bucks.
I also understand doing what you can to get the doors open and doing a rebuild after you get some money rolling in. LOL
Good luck on your projects.
PS if you guys call Tom Nudo at Prime Panels, tell him I referred you, it may get you a slightly better price or it may not but it's worth a try.