That's pretty much the only way to go without going into serious wiring. A power inverter, or a static inverter (yes, I happen to have one of those government surplus badasses laying around :chuckles: ) is pretty much the only way to go.

At cruise, this setup won't be much of a problem because your alternator will be producing a constant 13-14 volts. At idle or parked, this will be way to much current draw for your poor little battery.