First thing you need to do is to load the RV up exactly as it would be while you're traveling, and run it over a scale to find out the REAL weight - the UVW listed on any paperwork is nothing more than "science fiction". Once you know what the rig really weighs, you subtract that number from the GCWR.
If the result of that math is more than 3500 your next step is to inspect the hitch receiver and chassis. The 3500 lb tow rating is most likely set by the rating of the receiver. That receiver was installed based on calculations done by the vehicle's designers, along with a team of lawyers. Is the receiver bolted on to the chassis frame rails, or is it bolted on to some frame rail extensions installed by the coach builder? If the former, then you MAY be able to replace it with a 5000 lb rated receiver, if the weight you calculated above allows it. If, on the other hand, it's bolted on to an extension then you'll need to know how strong that extension is before trying to increase the receiver's capability.
Or, you could just say "Numbers are for geeks" and go ahead and tow whatever you want...