Is 12V supply wire to USB outlets rated for 15A?

so your missing the most important thing.  how long is the wire?   depending on the length 15amps on a 12V wire could be anywhere from a 16ga wire to a 8ga wire.

so under 4 feet you can use a 16 ga, 4 to 10 feet a 14ga, 10 to 12 a 12ga, 12 to 16 a 10ga, and 16 to 24 8ga. 

most wire, except maybe cheep stuff will have the ga written on the outside of the insulation so you just need to pull enough wire to see the stamps.  if its cheep you can use a set of wire strippers to get an idea what size it is.

with rv's you trend to see a lot of daisy chaining so the wire will leave the fuse box and go to every light and end up at a 12V outlet.  rv's don't fall under the building code, and is the wild west.  you won't fund drawings and two rv's made on the same day by different shifts could have slight variations in how the wires are run, but newer stuff is better as they have [premade harnesses they just slip in place and wire up.  so really you need to turn every 12V thing on, then pull that fuse and see what all goes out.  that will tell you what is on that line.