This thing could be useful in our vans.  Can be mounted vertically
too.  I put a cigar lighter outlet on the cabinet wall at the rear of
the fridge cabinet.  Near the little square grille for fridge heat
release.  Something like this would have been another option for
there.  I would want a switch to turn off the voltmeter though.

http://www.banggood.com/Dual-USB-Adapter-Charger-Cigarette-Lighter-Sockets-Digital-Voltmeter-p-949446.html

Edward