The Kindle won't last through a multi-week trip on a single charge, and it might or might not trickle charge via USB (this definitely won't work if the Kindle is out of juice). So take the charger with you. See
Help! Powered Down Kindle in Mexico. Also, turn off Whispernet before you go.
I have an iLiad and a Kindle, and I use the Kindle for trips because the larger size of the iLiad makes it harder to deal with. The iLiad does come with a travel charger kit, which is good because it will need recharging even on short trips.