A friend and I did an across-country drive using a similar size Penske truck, with an attached flat-bed trailer that was carrying a small car (Honda Prelude). Our journey was from Ohio to southern California.
He did all the driving (thank Ghu!) and we had heavy rain for the first 2-3 days. We only did about 8 hours a day and paused for bit in Phoenix to visit a friend. Anyway, the Penske truck did terribly on uphill grades. It didn't have as much power as a diesel truck does, so we would be going like 40 mph at most up the hills. After a harrowing trip through Albuquerque at rush hour (fun!), we dropped off the trailer in Flagstaff and I drove the car the rest of the way.
The truck was not packed to the ceiling although it had a full complement of furniture (no appliances) and a bunch of books and comic books which added weight. That may have affected performance.
Anyway, if the truck is going to be packed full, and the trailer is carrying a big car or truck, you may want to find the flattest route you can.