E-450 chassis is rated 20,000 pounds GCWR (at least) and at that vintage GVWR was 14,050 pounds. Ford specified a "tow rating" of 5000 pounds, which still puts you almost 1000 pounds below GCWR if you tow with the motorhome loaded to GVWR.
Clearly, the 3500 pound tow rating comes from Fleetwood, the final vehicle manufacturer. This lower limit is either because of the hitch installed, or because of something Fleetwood did extending the frame (or building the house beyond the frame).
You can probably get back to the 5000 pounds Ford says that chassis can tow, but you need to have someone knowledgeable to check out that hitch, and how it is installed.