A friend of mine has found Valerian root to act as mild sedative to prevent car sickness while travelling with his dog, who is now under my care. I should think this would help her relax if he/she's feeling stressed. Best advice says it should be given in small doses over a number of days surrounding the event, which in your case would be more suitable as the exact timing of the event can't be predicted. I consider the animal's well-being as I would my own so I try to give mine the same conditions as I give myself, even if this means sharing a bed. I assume you already use a topical tick and flea treatment such as Advantix, but if not, I would consider this essential, especially when travelling on the continent.
If she's been with you since a pup and you haven't experienced thunder together I'm guessing she's quite young? I think gentle exposure where possible and consistent reassurance from you should get her used to these kind of stressors. Dogs learn as humans do; quickly when young, but less so once response patterns have been wired in. But I believe that if the dog is generally of a resilient nature all stress responses can be overcome. One thing I've learnt from my relatively short time with dogs is that they, like people, and even more so as their language comprehension is limited, tend respond more to the emotional atmosphere than verbal commands. The best reassurance from you is a deep, relaxed smile. If you fuss too much when they are afraid it might reinforce that there is something to worry about. This comes from personal experience and a conversation with a vet/trainer who's experience lies with working dogs.
I'm sure if he/she's spent time out with you before it won't be too much of a step up. Good training.
Best wishes with your trip. Hope to hear the results and/or decision you make.
