Over 4000#s, the motor is probably going to be a weaker link than the chassis, depending on the length of the coach and the frame rail extensions bolted onto the main frame. Obviously a 37' with long extensions will create some serious and scary leverage if the tail tries to wag the dog, but my 30' has towed a race trailer that weighs maybe twice what you are considering. I wouldn't go much over 4000#s without headers, etc. and toad brakes, especially if you're going into the hills or are in a hurry. If you have a 502, a supercharger or a gearsplitter on a 28' or 30', maybe you can pull more. That said, my PO towed a 4000# Jeep for 60K miles all over the country with no brake hookup and stock manifolds. Did it for 10 years. I can't imagine how! It took me all of 2 days with a 4000# load to put the headers and brakes on. For me, you're good for 3500#s on a 30' coach with headers and brakes. Much more than that, you need a diesel or some fancy stuff. Should be able to do it, early-mid-90's Classic or Land Yacht, for under $30k. Maybe way under if you go back into the '80s.