Get yourself a 12V receptacle or several. Cyclegear.com, MotorcycleSuperstore.com, Ebay, Walmart, autoparts stores, and many other places have 12V receptacles made for weathering the elements on a moto. Install 1 or several. Make sure you use an appropriate fuse in the power wire, as close as practical to the source of power (battery, splice into bike's harness, etc.).
Find out what amperage draw your camera charger is rated at. Then find a little 120VAC-to-12VDC inverter that easily exceeds that amperage output. Find someplace safe from the elements for it. I usually put mine in my tankbag. Plug in your charger and you're all set.
Do NOT splice in a cut USB cord. USB inputs are NOT 12V like your typical moto's battery. USBs are typically 5V. Your USB devices could be damaged by 12V. Buy a 12V-to-USB adapter. I bought a nice little dual-output one for $8 at a truckstop. Walmart also has them. One output is like 2 amps. The other is 1 amp or so.
If you're not comfortable with wiring/crimping/soldering/dielectric-greasing/heat-shrinking/taping/testing/looming/zip-tying electrical circuits, watch some youtube videos of similar projects and have an experienced friend show you what they would do.
Once you get the hang of it, you can re-wire your entire bike and add auxiliary circuit-breaker blocks.