where can i purchase the proper plug in wiring for an RV 7 pin connector for my 2016 express 3500, and where is the connector located on the vehicle chassis.

 

The closest you can get to plug-in wiring is a T-connector for the 4-pin wiring - https://www.etrailer.com/Vehicle-Wir...cleid=20174370
The other 3 wires of the 7-pin connector (electric brake, auxiliary power and ground) you'll need to wire in manually.
Ground is easy, aux power can be conveniently grabbed from the interior fusebox located under the driver seat.

 

This is a basic of what you need to do to install a trailer brake controller.