Hey Max. Gas mileage varies a great deal depending on the rider, the bike and riding style. On my recent 1700 mile trip with my 1200S, I averaged close to 40 while on the road. I'm sure that number dropped when I was romping around on the curvy mountain roads. If you play, you gotta pay.
Enjoy your trip. I highly, highly recommend the roads and routes that I mentioned.
My bike is modified with a full aftermarket exhaust, UNI intake filters, and a Power Commander fuel module. It runs amazingly well.