Curt markets a frame mounted, bolt on hitch receiver that fits frames up to 72" wide. It will fit most Keystone RV's. Keep in mind that if you are concerned with voiding your one year Keystone warranty, the holes you drill in the frame to install a "bolt on receiver" will be a dead giveaway that the frame has been altered, so you probably won't win that one with Keystone. I will say that few people actually have a frame warranty and it's not really Keystone, but the frame manufacturer, usually Lippert, that has the final determination on whether to honor the warranty repairs.
You can view the Curt hitch here:
http://www.curtmfg.com/part/13703