Hi J.,
Try adding an extra pumping session or 2 each day before you go, hopefully you are using an electric pump, if not you should buy or rent one (hospitals often have ones you can rent). Make sure you drink plenty of fluids and eat regularly. I haven't used these, but you can try Mother's Milk Tea or fenugreek capsules, which are both supposed to increase your milk supply.
Most nation-wide hotel chains will provide a mini-fridge if you call ahead and tell them it is to store breast milk. On my first trip away, my daughter was 4 months, I bought a little cooler and fedexed the milk home. (I don't remember if the milk was frozen or just refrigerated). Just get a couple of large zip-locks and fill one with ice and double bag it (you can even tape the top). You can either check this as luggage, or fedex it overnight, you will not be able to carry it on the plane without your daughter (and even then, they may give you a hard time if you have too much).
You will probably have to supplement with formula, this is fine!!! You are doing a great job, express what you can, pump as much as you can while you are away to keep your supply up. Even if you don't end up keeping what you express while you are gone, if you are pumping you'll still be able to nurse when you get home. Ideally you would pump as many times as your daughter would nurse, but of course this is nearly impossible, so just pump as many times throughout the trip as you can.
Try to limit alcohol (if you are drinking any) and caffeine while you are traveling, and remember that just being on an airplane is very dehydrating. All of these things will decrease your supply.
Here's a story for your entertainment: On that first trip away from my daughter, I was working all day (14+ hours) in a hotel that was not where we were staying. I had no room in the hotel where I had any place to pump. I had to go out to the car, and pumped with my jacket spread across me for privacy, in the hotel parking lot with a security guard roaming around. The challenges of motherhood :)
Good luck, keep up the good work, but remember you are only human. You could try giving one formula bottle a day now, so that you can better space out the breast milk you have for your trip. Just make sure you still pump or nurse as many times as you normally would and/or add one additional pumping session before you go. (and don't worry if she doesn't want to drink the formula now, if she has to while you are away she will, she already drinks from a bottle)