Nine parks across four states, all of them the same trip.
State lines are the wrong tool here. This region is drawn on desert, and the desert does not stop at Arizona.
Death Valley and Joshua Tree are in California, which makes people expect to find them under Pacific. They belong here. Both sit in the Mojave, both are hot low desert, and both are on the same loop as the Arizona parks rather than the same loop as Yosemite. Big Bend and Guadalupe Mountains are in Texas, 500 miles from anything else in Texas, and reached from the west.
The practical test we used: could you reasonably drive these in one trip? For these nine, yes.
Splitting California between two regions is deliberate. It is a very tall state.