Re: Solar Vents
Swinada, look at the listing - the eBay units fit over a 3" hole. Judging from the photo, they appear to have 8 cells in the array, so I suspect it has a small ~5-volt computer-type fan inside. They are probably in "always on" configuration. When the sun shines, they exhaust. I wouldn't be at all afraid to try a couple, but I wouldn't expect the motors to last over 5 years. The flanged mounting would be easy to attach to a flat section of skoolie roof (if there is such a thing). A pair or 4 of them might insure air flow without a load on the electrical system.
I couldn't find the Costco units you pictured. I did see an attic exhaust with 38-volt motor and thermostatic off control for CDN $400. I think its a heavier-duty construction, and the photo shows them on flashing mounted into a shingled roof. Skoolie mounting may take some Skoolie ingenuity.
blkjck, making a reversing vent would probably require hacking the wiring. DC motors are known to run backward with reverse voltage, so a double-pole, double throw switch would probably do it. BUT (I know wiring but I'm not a pro on the inner workings of electric motors) I suspect if a motoris not designed to reverse, the bearings or brushes may not be designed to withstand running backwards. You will need air inlets in order to exhaust, anyway. Why not control the inlets instead of reversing the outlet?
The idea of solar vents is to take away heat when the sun is beating down. No batteries means no maintenance. There would be very little market if the vents required homeowners to climb the roof every year (or pay someone to do it) to check or replace the batteries. Their beauty is that they are "set and forget." If you want air exchanges when the sun isn't out, why not find efficient (low current) standard 12-volt power vents to run off of a solar-recharged house battery system? A lot more work and cost, yes, but the system can be up-sized to support other functions like LED lighting, etc.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.