This is a rough one. I guess I would take a three step approach. Before leaving I would vacuum the car to make sure I got rid of any excess dog hair that was there from before, I would brush and comb the dogs well to get rid of loose hair on them and I would take a light weight twin size bedspread and throw is over the back seat. (This one may be difficut because you need some manner of attaching it, tucking it in or whatever so it covers the seat and back of the seat and so the dogs won't have it on the floor in 2 seconds.) Then when you have a rest stop, you could remove the spread, and shake it out good. (A windy day helps.) I've used the bedspread technique over our couch and it works well, but I know the back seat of a car is a little different. Have fun on your trip.