Been eons since I was last at Palo Duro Canyon, and we didn't camp during that visit. Rangers could probably recommend the most popular (favorite) spots over the phone. You'd want one with a tree, if possible, as shade is at a premium there. But you'd also like to face the beautiful cliffs -- great sunrise/sunset views (it's a canyon, so you've got about a 75% chance of having a nice cliff view, assuming another RV isn't blocking it.
)