Doesn't have much to do with RVing, really. It is more about economics and time constraints.
When I was time-constrained, I would do a trip with 10-15 hours driving time in one day. Once I retired, that would be at least two days, or three, or four, exploring along the way. My nieces and nephews still do Michigan to Florida or Michigan to Outer Banks as a non-stop trip. If they want to visit California they fly.
Once retired and making seasonal moves, with months at the destination, the picture changes. My cousin has summer property in northern Michigan, winters in his RV in South Florida about six months. It is still 12-14 hours of driving, he will take a week or two to make the trip. When he was 30 and had to get back to his job, he would do it in one day.
You father-in-law is concerned about the economics of it, he got paid for miles, not for time. The kids have limited vacation time and want to maximize time at destination.
You need to stop worrying about what other people think about what your are doing. Different folks have different needs, which lead them to different uses of their time.
One more example, my wife and I, when she was still living, liked long days at sea, so we did a few positioning cruises, and round trips from the mainland to Pacific Islands. This means seven to ten "sea days" for one to two weeks at destination ports. If you have the time, this is nice, but if you have one week or two weeks for your vacation, you get to the destination as quickly as possible.
