If the power plug to the camera is USB it will be 5V, and there will be a 12 -> 5 V converter either in the plug you push into the dash lighter, or some have it in a little box a foot or 2 up the wire from the 12 V plug.
So you can use a multi channel USB charger to power the camera instead of the provided one, just have to be sure it is a good quality one and it can give at least 2.0 A on the port for the camera.
This way you have other USB plugs ready for charging other stuff in the car.
Also if you buy USB wires to bridge the gab between USB charger and camera on windscreen, make sure it is quality cobber USB cables and not cheap CCA ( Cobber Clad Aluminum ) cables with too much resistance in.
I personally missede getting rear ended as the cabel from the front of my car to the rear camera was a too long and too cheap one.