Tubes, tires, wheels in that order.
Learn how to adjust derailleurs, how to adjust the stops and cable tension, straighten a bent derailleur hanger.
Derailleurs are very robust, I've only had one fail from hitting a rock while MTBing, in over 20 years of riding.
They are available anywhere, are cheap, light, and easy to diagnose, adjust, or replace.
Bring extra chain links, chain tool, and know how to repair a broken chain.
Make sure your hubs and headset are freshly greased and adjusted before you go.
Douse your shifters with WD40 and lube your cables.
Check your bottom bracket for roughness or play.
Check your wheels for even spoke tension.
Check chain "stretch".
Do these things and you probably won't have a bit of trouble on your trip.