I can't speak to whether this has to do with the truck, your driving, the So Cal roads, or whatever. While I normally drive in So Cal, my major mileage is on long haul trips. I don't have experience yet with MPG in So Cal. Here is my gas mileage over the first 5,000 miles (doesn't include all fill-ups) based on calculations from fill to fill against mileage. Note that in a couple of cases I compare it to the dash estimate, which appears to me to be pretty meaningless.
I did increase my tire inflation cold to 37 all around just before the two best days. The truck dash advised that they got to about 40 hot.
The toll road you drove has lots of hills, as you note, some of them pretty steep. On the stretches I got the best mileage the weather was cool, so little to no AC (which really saps gas mileage per Consumer Reports), rolling hills (but not like the toll road), and very little traffic. Had two humans, one dog, and a light load of luggage.
The lower mileage I got was I think attributable to some serious uphill driving combined with lower tire pressure and very high outdoor head (100 or so) that had the AC working hard.