I would just wire up a power outlet and mount it on your bike. You can wire it directly to your battery (use in-line fuse) and mount it pretty much anywhere. Then you won't have to worry about it. At all. The only downside of wiring it directly is that it will still be hot when you turn off the bike so that if you leave stuff plugged in overnight you could conceivably run down your bike battery, and if you are wearing a lot of heated gear and extra lighting you may exceed your bike's charging system capacity. This is really the only good option if you plan on using GPS very much though.

I've tried to use solar chargers on motorcycle trips myself, and one problem is putting the panel somewhere that remains in direct sunlight. If you use your tank bag, your body will block the sun much of the time. Plus, if you are riding in mountains, as you mentioned, the trees will block the sun often as well. If you were riding across the plains or the southwest, a solar charger might work ok.

You might want to look at something like a Verbatim AA Power Pack charger that runs on AA batteries. You can stick a dozen AA batteries in your gear without taking up too much space, and you will not have to depend on direct sunlight. Plus they are cheap, less than $20 (plus all those batteries).

Have fun and ride safe. I wish I were going on a 2 week ride!