Every gas Winnebago that I've seen from that era had no more than a 3,500lb hitch on it. The Chevy and Ford chassis are similar in their ratings -- I think the Chevy is something like 14,000 GVWR and 20,000 GCWR and the Ford is 17,000ish GVWR and 23,000 GCWR. The GVWR should be on a placard by the driver seat, but they make the GCWR something that you need to scour the Internet for.
I'll leave the legalities to you, but something that they weren't very good at when they put the motorhome on the frame was going the extra mile to make sure everything was nice and strong. Typically, the frame rails that GM or Ford put on the chassis are not long enough for the house, so the RV builder puts frame extensions on. Since those extensions don't need to hold a whole lot, they put some extra C-channel at the end of the frame rails, throw a small plate over the outside joint, and do a one-cheek weld job. When you hook up some weight to the hitch (which is bolted to the extensions, not the actual frame), you build a giant heavy see-saw that will apply some serious leverage to that joint. Worst-case scenario involves the frame extensions coming completely off and the trailer taking the rear cap of the RV in a messy divorce. More likely is that the welds will start to fail and the rear cap will sag, which is pretty bad.
That's the biggest reason I see to pay close attention to their 3,500lb hitch limit. Just to make extra sure, I cut a couple bits of c-channel and bolted them to the insides of the frame and extensions and then attached the hitch through that as well. The reinforcement transfers any force that the trailer puts on the extensions through to the actual frame. Total cost was about $50 and 6 hours, but my hitch is really solid now.