I think you're living a bit close to the edge. Besides over loading the hitch there's also concern for the coach frame from the rear axle back.
Another concern is where you contact the dolly safety chains. Normally most of us just connect to the points provided on the hitch but with the overloading of the hitch if it fails or pulls lose from the coach the dolly and car are going somewhere you don't want them to be.
Either get a lighter toad, a newer coach or replace the hitch and beef up the rear frame by boxing it in, additional cross members and a good lawyer on a retainer.