Traveling from Western PA to central Florida takes us about 18 hours total, including all stops, about 1010 miles. We leave at 8am and stop at Fort Mill SC KOA around 5pm. About 9 hours to get to SC, and then another 9 hours to get to FL the next day. I get tired after 9 hours of driving the RV.
As noted by others, we generally leave early enough in the morning so we can stop before it gets dark on virtually all our trips.
The longest driving I did in the RV was 11 hours, mostly on Interstates. I was really beat after that.
So, when planning for future trips, I attempt to limit driving to no more than 9 hours.