Are there rest stops along interstate 10 where donkeys could be taken out of their trailer and led around for a while?

Yes, you can unload in the parking lot and then move them onto the grass, the field, or outside the fences, should you choose to do so.

I would call Riverside County Highway Patrol and ask all they could do is say no

I've never seen an area designated for livestock at the rest stops.

Yes....

I think so, friend