I feel like I'm reading this as 1/2 a long-haul of 1,200 miles / 17 hours and 1/2 as a multiple day trip:
Long haul/one shot:
I did Maryland to Florida which was right about 900 miles in a truck + fifth wheel. We had myself, my wife, a 8 year old and a 6 month old. We left between 10pm and 11pm and arrived about 6pm the next day. I slept the day before so that the wife and kids could sleep overnight.
It took us just about 20 hours once we factored in stops (breakfast and lunch and fuel/potty/diaper stops). The fuel stops were good for stretching legs and expending some pent up energy.
Doable, but painful. I needed a night/day to recover from it.
At 1,200 miles in 17 hours, you're average overall speed is 70+ mph. That doesn't account for any stops whatsoever. I assume your stops may be less frequent with a motorhome.
Multiple day trip
Our 6 month old was mostly fine. He did have a couple of 1+ hour jaunts where he lost his crap and cried for a long period of time. During the trip coming back from Disney, the traffic was buggered and he decided he was done being cooped up (after a day of being in a stroller at the park) and lost it. We went to a grocery store to let everyone burn off some energy. One miserable part was on our way back- late in the evening, dark, had pre-picked a campground that was apparently further than we wanted to drive. My wife ended up sitting back with him- he was crying, she was crying, it wasn't pleasant. Yogurt was the fix - calmed him down enough to stop crying. (It wasn't the cause- we had dinner.)
Remember, even if they're not walking much- doesn't mean that they don't want to move about.
I understand being rushed for vacation time and making time on the road- but remember, the little folks need their time, too.