The most flexible homeschool program for middle school aged children would be unschooling where children learn just exactly what they want to learn. They would be learning by experience and if you are traveling, that is just what you would be doing. Offer them journals, digital cameras and other tools for recording their experiences. Put them in charge of budgeting the expenses, anticipating tolls, hotel fees, tips etc. Before the trip, ask them to choose books on tape, language tapes etc to play in the car while traveling and use these to learn together as a whole family.
If you feel that you need to get credits toward highschool, consider Algers Learning Center in Washington State. It is a school for unschoolers and can be used by homeschoolers throughout the world. They have several programs. One just keeps track of credits you as a parent assign. Another has the student meet over the phone with a teacher to develop a curriculum. At the end of high school, Algers Learning Center would grant a high school diploma.