For my part, I would have the service done before I leave including the valve adjustment since you are unsure if it was done at 10,000 and you will be on your trip at 20,000. I'd do an oil and filter change, complete service including rear drive fluid change, radiator flush and change if not done already. Also brake fluid change, check brake pads, change if necessary, check tires, etc. As far as luggage, take only, (in my opinion only), what you will need for four to five days max on the road and stay somewhere you can wash your clothing. Taking too much crap is a burden and the extra weight is only that, extra weight for you to haul around with you. Take a tire plug kit, tool kit with minimum tools, maps or GPS and most of all, have fun!
Native One