I hope that the RV dealership was able to get back with you on a trailer hitch solution. This is a picture of the Hidden Hitch (brand) found on ETrailer.com website.
![](http://www.etrailer.com/Specialty-Trailer-Hitches/Hidden-Hitch/82200.html)
Basically you have the drivers and passenger side brackets to mount to the frame. The center section with the receiver already welded on fits in between the side brackets, and is bolted in place once holes are drilled into that section, and you are done. Installation time is about 2 hours.
Unless your frame extension is 34" wide, then the E-350 club wagon hitch will not fit the E-350 RV chassis. Like stated above, my E-450 chassis is 42" wide, so the passenger van hitch will not fit it.
This page has 5 hitches that will fit frames from 18" to 60" wide. Only the 3 on the bottom of the page offer a decent tow rating of 5,000 pounds, and are priced in the $200 - $240 range.
http://www.etrailer.com/dept-pg-Specialty_Trailer_Hitches-sf-RV_Hitch.aspxI hope that you find what you are looking for!
I would not be installing a hitch on your frame extension,(based on the pictures that you posted of the extension, it does not look all that strong) unless it is strengthened first. But if you are only going to tow 3,500 pounds, it might be OK as it is. Problem is if you fatigue crack the frame extension, the only way you will find out about it is when the rear cap starts to pull apart, or something expensive to repair starts falling apart. It would be better to mount the stronger hitch to the factory frame section, then use a 18" long hitch extension or a very long receiver hitch.
That is why I am going with a custom made hitch, welded up and then bolted to the factory frame.
Fred.