The issue I found with the Transit TS series is you can't use a regular Transit 350HD hitch due to the frame extenders. Even if you could mount a "off the shelf" commercially available hitch it would be some distance "in" from the rear bumper...
The best solution I could find was to use the Coachmen hitch they install on the Orion series Transit 350HD based motor homes. But there is a "gotcha" in that plan as well...
The Orion's do not have the same fiberglass rear bumper style as the Forest Rivers. The Orion use a traditional square tube bumper with a straight rear wall / cap. When I looked at adding the Orion hitch it looked like I would have needed to cut out the license plate area on the TS bumper. That
should allow access to the Coachmen hitch. I decided against proceeding so I never mocked it up to confirm proper fitment or if the "drop" was acceptable.
You can order an Orion hitch through any FR dealer, you do not have to go to a Coachmen dealership. I'd order all the mounting hardware as well as it maybe some off the wall sizes...
The other option is to have a local hitch company make a "custom" hitch for you. I think you'll still need to cut out the license plate section of the TS rear bumper for access going this route...
Good luck!