It's hard. I have been in this situation LOTS in the last couple of years. I find I only do well when I am mentally ON PLAN, like fully committed. As soon as I give myself permission to go off plan, I am all over the place.
Not being on your own time and being at the mercy of other people's choices for meals and restaurants is a challenge. Hopefully you can tell your family that you are committed to eating healthier and need some support.
When I have a similar situation, I always bring as much of my "own" food as I can. And if possible, I go shopping as soon as I arrive at my destination to make sure I have a few things on hand. Yogurt, fruit, nuts, tuna, salad greens, hummus and veggies, lowfat cheese, etc. etc.
Plan some snacks for the travel day(s) so you are not at the mercy of fast food. If you think fast food is in your future, there is a list around here that shows the best fast food choices for SBD.
If you will be eating out lots, just modify your orders to be as healthy as possible. Of course i have been places where this is impossible (the healthiest thing is cooked-to-mush green beans or iceberg lettuce) but oh well. For breakfasts, you can almost always find plain scrambled eggs and plain oatmeal, which is my go-to restaurant breakfast. Add fruit and/or plain yogurt if they have it. For lunches a salad with dressing on the side and some protein is what I aim for, but as I mentioned before often there is only iceberg lettuce. Just do the best you can with what is offered and hopefully don't starve since you will have a stash of food at home!
If you will have internet access, check in here and join an exercise challenge to help you get lots of walking in. As the previous poster mentioned, you can also do body-weight exercises such as squats/lunges/pushups/planks anywhere and these are great, challenging full-body exercises. I have also brought hand weights with a DVD (30-Day Shred, which only takes about 20 minutes and kicks my butt every time! I found I could usually sneak away from the family for this and no one noticed).