Effectively you have 12 days. After picking up the rental on the 1st day the rest of the day will be spent supplying it. The return day is just that.
Our very first RV trip was a 15 day rental. 2 families. 2 units. We picked up & dropped off in Denver & did a loop that included Pikes Peak, Grand Canyon, Vegas, Bryce Canyon, Rocky Mtn NP & a couple shopping days N of Denver. A much shorter miles but longer time than you are suggesting.
We had a blast but got back on the plane in Denver busted.
You might do well to heed to advice of those who suggest limiting this trip to the northern portion of it. Save the Grand Canyon for another time. It is not going anywhere.