The Diamond Mountain Casino, in Susanville, CA, is listed in the American Casino Guide as allowing free overnight RV parking. And they do - we stayed there for one night recently, but wouldn't necessarily try it again.
The parking lot is fairly small; on a Tuesday, we found enough space for our big 5th wheel but it might be difficult or impossible on a weekend. There isn't a designated RV parking area - you just hope to find enough empty adjacent spaces to maneuver into. The parking lot is also not what you'd call level. In the best spot we could find, we still had to put 3 leveling boards under the tires on one side.
On the upside, we ate at their Grill restaurant, and the food was decent. The security folks, who gave us the OK to park overnight, were pleasant to deal with, and I did see security patrols at night.
The video poker paytables weren't very good. They have some table games, but I didn't take much notice of them.