Call your own pharmacy at least 10 days before you are leaving. As long as it is not a controlled substance, they will be able to tell you exactly when the insurance will pay for the rx. Allow two extra business days before you have to leave in case something unexpected happens. If the rx can't be filled in time for your trip, ask the pharmacy to get a vacation override for you. They will have to call your insurance company. With enough notice any decent pharmacy will do this. It does help if you also call the insurance company using the phone number on your rx insurance card.
As with any request from any pharmacy if the staff person at the pharmacy is not helpful or doesn't understand your request, ask to speak to the pharmacist.
Make sure you get the name and extension number of everyone at the insurance company and the name of the staff person at the pharmacy as well as the time and date of your call.
The most important thing is not to wait until the last minute.
PM me if you have any questions.
Have a great trip.
Gobo2, RPh