If you lose power, the bigger concern is getting hermaphrodites. When the lights are off, you don't really need ventilation, as the plants don't make stink in the dark. Plants seem stinkiest shortly after lights on.
really dude? no offense but you are way off here, wort case if he lost power for TOO long the plants would go into flowering (assuming he is in veg stage when this happens)
and the plants smell the same day and night, they just plain stink. reason they seem more potent after lights go out is probally due to having your fan setup on same timer as lights. just a thought.
providiny your cab is airtight, a couple of 12v CPU fans would work nicely hooked up to a good car battery, wich is also a 12v battery, just have to rig something to replace the inline fan and put in a couple CPU fans real quick, im sure you could think of something to be able to quick change them in a emergency, they wont stink you out that fast but if its really a issue have some air freshner handy for the time it takes you to rig the new fans.
if you are gone part of the day and woried about outtage when your not home, get the backup thing the OP was talking about that way if you lose power it will auto kick on and last till you get home and can figure out a more permanent fix depending on how long you think you will be out of power.