Even though you have a class 3 hitch, your MH is still rated for 3500 lbs. If you have the FWD version, you are 10% over the rated tow capacity. Can you get away with it? Probably. Will your engine or transmission blow up? Unlikely if they are in decent shape now. But Fleetwood derated the towing capacity for a reason. If you want to risk whatever failure Fleetwood anticipated with a greater tow load, then go ahead. I doubt that I would were I in your shoes.