Welcome to the forum. In response to your original question, I personally don?t think the trip to Disney World you were considering is unreasonable at all. As suggested by other responders to your post, two or three trial runs close to home would benefit your setup times and how the TT you buy and your tow vehicle handles.
I don?t have nearly as much experience as many of the other folks on this forum, but because of that I can still remember the nervousness I felt when we took our trailer our the first few times....and all the mistakes we made. Lol!
One thing I learned right off the bat was all miles are not created equal. When I plan a trip, instead of deciding on how many miles I need to go per day, I look at what is in between my starting point and my final destination. Big cities, heavy traffic, mountains, bad weather......all these things affect my driving plan and the amount of fatigue I experience. I usually have alternate routes planned for ?just in case scenarios?. For example, if I was planning the trip you originally posted about I would have planned stopping the first day just short of Atlanta and buzzing through there early the next morning before rush hour. That?s because I know how the traffic in Atlanta can be. Next, I would have made another stop short of Disney World....like Valdosta, Lake City, somewhere like that. The last day you can leisurely cruise into Disney World with plenty of time to get set up and still have some time to explore the park if you wanted. On trips home, seems like I?m out of vacation mode and always seem to be able to make better time and travel further for some reason.
Most of our trips are out west where the traffic is lighter and the roads get nice and straight. Wind can be an issue for sure, but in general it is a much more relaxing driving experience than anywhere in the east or southeast. But there are tons of really great campgrounds and things to see in those areas of the county, so we just sometimes suck it up and deal with it. ?
Happy travels to you and your family.