Hey K.,
My husband and I made this trip 2 summers ago with our (then)2yr old, 6mo old (boys) and 4 yr old lab. Our strategy was to take it in 2 days, stop at rest stops every 2-3 hrs (unless they were asleep or interested in something) We also did not have the built in DVD player and bought a portable Sony. We did have the removeable headreasts and strapped the portable case between the seats--I'm don't remember exactly how we hooked the bottom--but it worked out great and kept the 2 yr old entertained (the dog entertained the 6mo old)
The actual trip was pleasurable--a lot of driving but lots of fun. With lots of stops we were able to keep the kids and dog well exercised. The 6 mo old drank bottles in the car, ate solids and played on a blanket while everyone else ate and ran, ran, ran...with a ball to chase during the stops. We kept an open mind and really just had a go with the flow attitude.
I packed an overnight bag for the boys and one for us so we only had 2 small bags to carry into and out of our hotel. Also I packed sandwiches & snacks for the cooler so everything stayed as much as normal as possible. Diapers/pull-ups and wipes were kept by the back door of our suburban--easily accessable during stops
We are going again this year--by plane as we don't have as much time for vacation this year.
Our sony dvd player worked very well and into the next year until our 2 yr old (who always wanted to carry it around the house while watching movies) dropped it about 5 times and the last time was on the tile floor and it did not work anymore. We have just replaced it with a Durabrand ?? (recommended by the salesman at wal-mart) because it had an extra large screen but is a tight fit into the carrying case. Only time will tell...
Good luck with your trip!