If I remember correctly, many hitches clearly state NO welding and NO drilling. They are certifed at factory and insurance companies love them "as per original factory build" as well. Not too sure why trailer shop wants to weld or drill onto a store bought. To me, very bad idea to drill or weld onto existing hitch. Expecially from a legal view.
To mount the 7-pin connector (with is spring loaded lid) on my prevous van, I visited my local welding shop, got a small thick steel plate from their scap pile and they created a round hole (for the 7-pin plug) to fit into it. At home, I then drilled 4 x small holes in it. Then, self tapper scewed that plate to the backside of the rear bumper - NOT onto the hitch. For a picture, surf: http://i178.photobucket.com/albums/w...r-Lights-3.jpg
If wondering, I did the same type of 7-pin connector install on my wife's mini-van as well. re: Screwed the 7-pin connector on the vehicle's rear bumper (not onto the hitch). For pictures, surf: http://i178.photobucket.com/albums/w...xtralights.jpg
IMO, the 7-pin connector should be mounted high up (under the rear bumper) instead of on the hitch - where there's more salt and road junk.
Forgot to mention... To convert 7-pin to 4-pin, many auto/trailer parts stores well a simple plug-in converter. Thus, no need for both 7-pin and 4-pin connectors under the rear bumper area. Simply install 1 x 7-pin connector and when needed, use the 7 to 4 pin coverter. Less exposed pins back there (where dirt loves to collect), the better. That's why 7-pin's come with a spring loaded dirt protector cover.
Hope this helps....