Miles per day/hours on the road

I used to do some long distance motorcycling and the general rule for planning a normal motorcycle trip was plan your first day to be the longest, but usually by day 3 or 4 cut that mileage in half. Obviously its a little different in a car or rv, but your mileage will drop as the trip goes on. Having said that, I have done 600 miles in a day with 2 young kids. they do travel better in a motorhome than a car, esp if there is another adult along to help out. But I agree I wouldnt want to do more than 2 days like that without a rest break. We are going to Disney in March, 1200 miles each way and are planning 2 days out and 3 back.