Hello and welcome to the RTA forums, glad you are enjoying them !

You are right when you say that 9-10 hours in a car is a lot for anyone, however it is manageable with Lot's of regular breaks. The problem I see is that are a lot of your days in the 'real world' and away from mapping programs generated by computers that need no time for rest, food or bathroom breaks to fill with gas or other human factors like traffic and construction delays are nearer to 11 or 12 hour days. All the day's are doable but on a multi day trip, day after day it is pushing the limits.

For example, from El Paso to Grand canyon and then to Sequoia NP are 2 extreme days on the road and you will arrive late and have to leave early while setting up and breaking down camp without the time or energy to appreciate the Canyon.

If you could add a couple of days you could make it a great experience but as it is it will be very 'work like' without time to enjoy the wonderful places you are stopping the night.