We've done what you're doing - leave while DD would sleep for much of the way. One thing we have tried successfully was leaving about 5pm. We would only have to amuse DD for a couple of hours in the car - then stop for dinner and then put her in her PJs and she'd go to sleep (soon enough) in the car. That would mean we could either arrive at our accommodation late that night, or drive through the night. DP and I would have a nap in the afternoon and/or a sleep-in in the morning before leaving so we were refreshed enough for the trip. Don't know how that would work for you, but boy it made a difference not having to stop after 7pm unelss we had to! It was worth one late night in the car.