I called the casino today, and was told that RV parking is permitted in the lot behind (to the north of) the parking structure. If you zoom on the satellite imagery on the Google map, you can actually see RVs there.
My daughter works at Casino Morongo, in Cabazon (between Palm Springs and Banning on the I-10). When I went to pick her up tonight, since her car's in the shop, I noticed they have a special RV parking lot. She says people stay overnight there all the time.
A little research led me to this: at http://www.rvdumps.com/ca.htm there's this listing for Casino Morongo:
"Dump station is at the Shell gas station owned by Casino Morongo, 49750 Seminole Dr, 800-252-4499, $15 fee, free overnight parking allowed in casino parking lot."