I would put in something like this:
...and tap into the existing cig lighter wiring.

Amazon.com: 12V USB Outlet 2 Pack, Dual Quick Charge 3.0 12V Socket USB Charger with LED Voltmeter and Power Switch, Waterproof Aluminum Car Charger Adapter for RV Marine Motorcycle Truck Golf Cart RV etc. : Cell Phones & Accessories
Amazon.com: 12V USB Outlet 2 Pack, Dual Quick Charge 3.0 12V Socket USB Charger with LED Voltmeter and Power Switch, Waterproof Aluminum Car Charger Adapter for RV Marine Motorcycle Truck Golf Cart RV etc. : Cell Phones & Accessories
www.amazon.com
...and tap into the existing cig lighter wiring.