Lear: I did see that site, and I built my fan off that idea. I'm using a 12v dc fan that's 'sparkless' (found on a marine forum)
It's just just a tad smaller than a computer power supply fan. It does look suspiciously like one too.
4x4: I think blowing in world be a very bad idea. Instead of pulling the trace gas out, i would be pushing it into an otherwise closed compartment. Even with a proper hole on the floor of the battery compertment, hydrogen gas is lighter than air by a good bit.
I think all a fan blowing in will do is help to keep the hydrogen -inside- the camper.