You have to stop often and walk. We are prone to blood clots in our legs. That's what my Dr told me if I travel. She said every hour.
I just got back from a trip by bus! It was a 12 hr drive there, and 12 hr back here! What I did, was get up every couple of hours and walk to the restroom and back, I would exercise my feet by doing circular motion!
My doctor said no more than 2 hours of sitting. If you stop and get out and move around you should be fine.
I heard is not good sittin down for longest or be standing up for longest
Talk to your Dr about the planned trip let him know what you guys will be doing and see what he says. :)