We just got back from driving to WDW from SW Michigan. Agree that you should keep bathroom and gas stops short ... but we tend to stop whenever anyone has to go without any pressure of "We just stopped!!! We don't want to stop again so soon!!!" Even if someone doesn't think they have to go, it is amazing how cramped up you can get from sitting for a couple of hours!
We keep a cooler and food accessible in the van so don't make any restaurant stops.
We ALWAYS make sure we have had a bathroom stop and a full (or mostly full) tank of gas before going through any large cities. You never know when you are going to get stuck in traffic (1-1/2 hrs to get through Nashville).
Have a good time and try to keep to a reasonable speed so you don't end up with a police stop -- which would really put you behind schedule!!!