One of the top casino cafes in Las Vegas that serves a decent cut of Prime Rib is Jerry's Nuggat on the north side of Fremont Street.
Here's a shot of one of the smaller cuts that ran about $14 on the menu and included salad and potato.
They serve larger cuts but if you order the biggest slice, the waitress warns you that almost no one has ever been able to finish it.
This "small cut" in the photo was half eaten by the time I took the picture.