The closer you are to the southern border, the more checkpoints you will find.
I can confirm you will encounter them on I8, and Hwy 95 between Yuma AZ and Quartzite.
In the sand dunes area just west of the Colorado River, you will see border patrol
vehicles all along the south side of the freeway and even in the rest area.
I travel in my '90 Westy between northern Arizona and San Diego with my big black Lab.
The back curtains are always closed, but they usually just wave me through, except for
one time the guy wanted to open the back hatch. I said I have a dog back there and
stuff piled against the hatch and got out and opened it myself. He wasn't happy but
that was the end of it.
Always happy to view these places in my rear view mirror.
Jerry