A lot depends on the weight of the motorcycle that you plan to strap on the back. With a 4500 tow capacity, you can probably not add any more than 450-500 pounds TOTAL hanging on the back of your coach, or the approximate maximum tongue weight for that chassis. This will allow you to take some of the small motorcycles/scooters, but certainly not one of the big Harley or Indian bikes.
Adding a lift or tow bar to the rear of the coach will still be limited by the gross COMBINED vehicle weight, which is the manufacturers recommendations on the total weight of your vehicle, when loaded, and the weight of whatever you are towing. I think the numbers may add up where you could add a motorcycle lift and a LIGHTWEIGHT motorcycle and still have the ability to use a tow bar and tow a vehicle in the 3000-3500 pound range.