There's no USBs. The only thing I can recommend is
1) Prepping your phone before you leave for the trip. Upload all your photos and videos to your computer then delete them from your phone to make space.
2) Purchase extra space from the cloud. It should be available in the storage area of your iphone settings. You can basically go on a month-to-month plan. I pay $1 a month for more storage. Once you take videos, send it to the cloud and delete it off your immediate phone.
3) Delete unnecessary apps. Also on the storage settings is the ability to check how much storage a particular app is using. If you don't need it, delete it.
I know how stressful it can be. I had an iphone a few years ago that was throttling on storage. I purposely bought an iPhone with storage so high that I'd never need to worry (or possibly fill) it again.