Stop at different places for bathroom visits and gas fill-ups - Stop first at rest areas that have multiple bathrooms and can accommodate several girls and adults at one time to move things faster. Then stop at a gas station and those girls who thought they didn't have to use the rest room before (there's always one!) will have a second chance. While this sounds like a lot more stops it actually moves faster than trying to have everyone use the one rest room at the gas station and then having to stop again further down the road. 

Make sure that all girls call home when you arrive at the hotel - Even better would be to have them call home each night.

Keep track of receipts and costs - It is a good idea to do this on a daily basis and have an envelope for each day.

Take lots of pictures  

Have the girls send postcards to family, friends and the people who helped them get there