Jaime, sounds like a fun trip/adventure. www.borrowlenses.com
is a great company to deal with, I have used them twice and been very happy with them. If it were me I would look into somthing like a Canon 24-105 f/4 L to add to what you are already taking. It will work well on the XTI crop sensor, My guess is once you put it on your camrea it will stay on most of the trip. A 50mm 1.8 would also be nice for low light shooting, and there pretty cheap, you could pick up a good used one for 80 to 90 bucks.
Post some pic's when you get back.