I can do 500 miles a day for two days. That's 9-10 hours driving for a 12-13 hour day. In the RV, it is a 14-15 hour day, with setting up and breaking camp.
I've driven 800 mile days (16 hours behind the wheel, 18-20 hours awake) for single days or last day of a longer trip. It helps then to have alternate drivers, but I can't usually sleep with someone else driving. Ican't even sleep on a plane with someone else flying.
Any longer distance road trip, I try to limit my driving time to 6-8 hours a day, 300-400 miles for most of the rural highways I use.
At 70, I don't drive nearly as far as I did at 20.