I would start with new belts and hoses; carry extra common items if they aren’t prohibitive like thermostat, radiator cap, perhaps alternator, water pump. Starter doesn’t get used much considering the length of trip. Common marker and signal lamps and fuses, couple extras. A spare to match what you’re running. A tube of good gasket maker/repair. A large packing box or other big cardboard to lie or kneel on if you need to change tire or work beneath the vehicle, ramps if you have room.
You can’t bring every foreseeable item of course. Some of these things, like starters, alternators, etc.... if you have a bunch of miles on them, maybe change em out ahead of time so they’re fresh, especially if they happen to be a real pita to work on.... consider that you probably won’t have better conditions along the road.
The good thing is that today’s machines tend to be so highly reliable that problems are unusual, especially if you’re just doing consistent, steady driving.
