You are screwed....LOL Just kidding!
I have traveled quite a bit while OP and didn't lose weight, but didn't gain either when I returned. I did meal replacement bars for breakfasts...Subway salads for lunch (if you can locate one), and dinner out of my choice....while still being somewhat careful
Still did my HNS and supplements daily also.
I packed my HNS in their box in my suitcase...didn't take in a carry-on. Too darn expensive to get taken away!! LOL Just drank one before my flight and after...etc..
One thing that worked out great for me was I have a water jug I carry constantly...and at the airport, every restaurant was willing to fill it up for me with ice and water...even if I purchased nothing from them. This way, I was sure to get my water in!
Travelling is sorta hard, but doable. Try to make your goal realistic for yourself. If your goal is to simply "maintain" your weight while you are away, it is much less overwhelming than trying to lose any while on a trip.
Hope that helps!