Consider an 115v-to-12v AC/DC inverter/converter power adapter..
https://duckduckgo.com/?q=auto+115v+to+12v+inverter+converter&t=ffab&ia=web
To be able to match needs of the portable Mac, simply use an Apple
adapter original equipment; and plug that in, as final link. +Consider,
extra battery backup, to power the Mac portable; without power loss.
(To avoid spikes, fluctuations, use filtered power; such as UPS battery.)
High-quality power supply makers, had DC-to-DC converters for RV/Boat
&/or Airplane. ~ The idea would be to match portable Mac voltage demands
from the Vehicle DC output. [Or, not use an inverter AC-to-DC, plus transformer.]
Ones I'd seen by Lind, matched iBook/PowerBook G4 exact output from vehicle.
For a larger motorhome, boat, plane; a complete DC-to-DC custom wire harness
with built-in safeguards; and secondary battery-backups, could be the ticket.
Yet, for a few days per decade, a simple AC120v-to-DC12v inverter may do OK.
Just that wasteful idea, has to go from DC12v to 120AC; then back to DC again.
Good luck & happy trails!
🌞🌿⛵️🎣🌻🐢⛄️🌤🌎