If you're looking for a spot in the Oklahoma City area, the Firelake Grand Casino at exit 178 on I-40, near Shawnee, has added RV parking with 30/50 amp and water. It's not glamorous, but it's free! The spaces are marked with parking medians along both sides, but they were wide enough for our triple-slide 5th wheel and the water pressure was great!
An added plus, there's a stream that runs behind the sites and the area along it is currently being landscaped. Due to recent high water in the area, it's currently full of fish! Our grandson caught three in a matter of about 10 minutes, using "free" bait - the crickets that are in abundance this year.
The casino itself is very nice, more like Las Vegas casinos than we've found most Indian casinos to be. The Steakhouse is awesome and the buffet was so-so.