The best way to do this is to run new wiring to the 12v fuse panel, but this is only an option if you have openings on your fuse block. Otherwise, it's perfectly fine to tap into an existing wire, but I would only do this if the wire I was tapping into was a heavy gauge wire like 12 AWG. In my present camper I've done both. Yesterday, I installed a Blue Sea 360 Panel that has a 12v outlet on one side and a dual USB charging port on the other. I ran new 12 AWG wire to the 12v fuse panel for this new panel. The full details on the install are in my blog.