My 1957 Fire Engine gets 6.2 mpg on the road empty...I'll be amazed if you do much better than that.
For 12v take a battery jumper/booster pack with you. Most have power outlets to plug in your cell phone charger and they can be recharged along the way. In a pinch you can also use it to jump start the truck it won't hurt the starter motor, but will blow anything else that's turned on like light bulbs.
Have you thought about taking a CB radio. Truckers still use them and they could advise you on who to call if you need roadside assistance. Mine is hooked up and looks like a Fire radio. My trucks is to noisy to listen to music, so I listen to the CB.
Have Fun!
Mike B
