Last summer I did a 3,600 mile solo road trip across the western US - Washington, Idaho, Montana, Wyoming, South Dakota, Utah, and Oregon. 21 days and 18 of them camping at state parks or USFS campgrounds. I had with me the following electric "stuff" that required regular charging: cell/smartphone, tablet, headset, GPS, LED lantern, and MP3 player/radio. I succeeded in not having to buy any batteries, nor did I take a bunch with me. Rather I used the 12V adapter on my 2006 R1200RT and a Goal Zero solar panel and battery pack.
The solar panel fit conveniently in my clear vinyl map case on my tank bag. I would have the solar panel connected to either a device or the battery pack inside my tank bag. I found that while riding the solar panel was almost always in direct sunlight. I also used the on board 12V adapter connected to a device when riding. When I would stop for the day I would set up the solar panel in a sunny spot and again charge one of my devices. I would also connect the battery pack to charge a device. I never needed to connect, even temporarily, to a 120V outlet. I was able to keep my stuff charged throughout the trip. I just rotated my various devices based on their charge level to something that would charge it, e.g., solar panel, battery pack, or 12V bike connector.
The devices I had were all low voltage and low amps. I don't think the solar panel or battery pack could have charged something like a laptop. Goal Zero makes bigger solar panels and battery charging packs but conveniently having them on a motorcycle is problematic. Perhaps a bigger solar panel could be mounted on top of a pannier or top case. Finally I packed pretty light trying to avoid multiple external bags and straps, although I did have an Ortlieb bag strapped to my BMR flatbed rack. This bag carried the big stuff like the tent, sleeping pad and bag, portable chair, and rain gear.
Thought I'd share this easy and environmentally friendly way of keeping your electronic devices charged.