A 1996 Fleetwood Bounder of your vintage Had the frame rails extended by Fleetwood during construction of the coach. This was to accommodate both the addition of the tag axle as well as the extra coach length. Depending on the quality and related engineering that went into that job it would cause me to adhere closely to that 3500 Lb. weight rating.
You could beef up the added frame rails and the manner of attachment to the original Ford rails as well as installing a 5000 Lb. or more rated hitch and properly rated attachments to the re-rated
frame rail extension. My guess that anything weighing more than the weight of your present setups rating would only serve to cause you more problems in the event of a mishap than you want or need.