Charging eBike Battery with Solar
A 100 watt solar panel is nominal watts under ideal situations. You'll be lucky to get 25 watts/hr tied to the back of your bike - and this would be only for a few hours of the day, from 11a to 3p. 

Given that your bike is likely using at least 100-200 watts/hr even on low pedal assist, you're only going to gain 5-10% more range over the entire day. From experience, even with low pedal assist, its highly unlikely you'll get anywhere near the maximum (or even middle) advertised range of the ebike. 

There is a further complication that  your battery management system won't allow you to charge while biking. Many if not most don't.  So you can only charge the spare pack and you'll have to figure out how to convert the solar panel to the right D.C. voltage and amperage for your charger. If you overcharge or overvolt, a poorly design lithium pack can go up in flames.   

It'd be much easier and safer logistically to just bring another spare battery pack or two. I assume you'll be pulling trailers and as a result your mileage will be even further reduced but it means you can carry more. Just load up your trailer with spare packs and bring multiple chargers. Forget solar, it's not worth the hassle. The one exception would be to use solar to charge your iPhone/iPad and other electronic devices -- it's relatively easy to find small solar chargers that have USB leads.