Long distance drive is excellent opportunity to flush the system using 6-8 hr radiator flush stuff..... first you gotta drain it until water runs clear, then add the radiator flush and take your trip. You can even repeat the procedure if its been long time since previous flushing.

Naturally bring some orange anti-freeze in the trunk for when you arrive at destination. Good place to purge the radiator flush out of the system might be 2 bit self-service car wash. Just make sure the engine is COLD whenever you douche the system, drain the radiator or add fresh coolant and water.

Btw, those backflush kits with inline garden hose fitting are not bad..... they can be moved around to flush the galleries in cylinder head, block, and heater core too. Another place you might be leakin coolant is at heater valve/ hose connections. And if you've got rust in the system it will probably dirty the walls on the expansion tank as well as clogging the heater core.