If you go to Google and type in tow rating for the E450 chassis you will come up with many links that show chassis specifications for that vehicle. I recently did the same thing for my RV chassis and was able to save the information because it was from Ford in PDF form. I believe the E450 has a gross combined weight rating of 23000#. That is RV + trailer. Then you will have to have the RV weighed in travel form to get you GVWR then subtract that number from the GCWR and that is the max you can tow. Plus the hitch and tires have to be rated for the weight you intend to tow as well.
Good luck