The standard receiver is proably 5k and the usual reason for that is the attachment points for the hitch to the chassis. Older gas chassis typically had extensions built onto the back for length, so the tail end where the receiver mounts isn't that strong. Maybe you can find a welder talented enough to beef it up for a heavier hitch, but I would not attempt it myself. Besides, you are going to want more hp and braking if you plan to tow anything that needs that much hitch.
Flush floor slideouts are a different design. It's not just a matter of turning a few adjustment bolts.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL