The simplest, and probably least expensive way to have 12 volts available is to install a small 12V battery like one for a garden tractor, and an onboard trickle charger. Just plug it in occasionally to keep the battery charged. Solid-state accessories like a normal radio/CD player, etc. (not some mega-watt booster) and power taps for cell phones, etc. use very little current. An occasional recharge overnight will keep the auxiliary battery topped out with no problems.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!