I drove across country with my 3 year old son from Seattle, WA to Atlanta, GA. It took us 5 days and 4 nights because we tried to break up the days and the amount of time we spent in the car. When traveling with a toddler I have a few suggestions that you may find useful. First, if he is potty trained, I highly suggest him wearing a Pull Up just in case. They sleep a lot on long car rides and sometimes it's easier having them wearing the Pull Up then it is to have to pull over and strip them down and clean them up and line their car seat with a towel and etc. Pull Over every couple of hours and let them out to run and to go potty and to stretch. Only if they're awake. If he's sleeping then just keep rolling!! Have plenty of snacks for him and keep the activities light and simple. Having a laptop or portable dvd player with you will help keep him occupied. Just make sure he's got head phones for it so that you aren't listing to Thomas the Train for hours on end! Also, sing along disks where again, he can have his head phones on are fun and it's cute to listen to them try and sing along! Coloring is ok, but keep it simple. Just a handful of washable crayons is perfect. Bringing along a few books to read to him is nice as well but I would not suggest beads or anything involving cutting or small pieces. Pack a few of his favorite toys that he can have easy access to while in the car as well. You will find that if he isn't sleeping or watching a movie or a sing a long dvd then he's going to be snacking or watching what's going on outside of the car. If you try and make it too complicated then he will get frustrated. Also, if you can leave really early in the morning I would suggest that you do that. Try and keep him up as late as possible and then, when you leave early in the morning he'll sleep most of the first part of your trip. Just remember to stop and let him out of the car and don't try too hard to keep him entertained. You will just end up exhausted with a very craby baby in the back seat to boot!
I can honestly tell you that my son did better traveling in the car across the country then I did. By the third day I was in tears and couldn't stand myself and he was happy as a clam in the backseat!! If you're planning on driving straight through then if possible I would leave at night or very early in the morning. Don't keep going if you and your husband are both tired. It's just too dangerous and not worth the risk. If it were me, I would leave a day early and try and make it half way there. Then stop and get a hotel for the evening and then finish the last half of the trip the next day. You will find that breaking it up like that is easier on all involved and well worth the added expense!