Before going on a long road trip that is more than a few hundred miles I go over the vehicle completely to make sure everything is working properly.
I check the coolant levels, transmission fluid levels, engine oil levels, check tire pressure, check tire condition, check the serpentine belt condition, check heater hoses, check radiator hoses etc.
I also pull the wheels and check the brake shoes and brake pads on my truck or check the brake pads on my car to make sure the brakes are in good shape and check for any potential brake fluid leaks.
I also check for any other fluid leaks such as coolant leaks, transmission fluid leaks, engine oil leaks etc.
Keep extra coolant in the trunk of the car and carry extra engine oil and transmission fluid just in case you need to add oil or other fluids.
After every 200 miles on the road I pull over to get gasoline and while I'm getting gasoline I also check the transmission fluid and oil levels to make sure they're full.