Being on the road, I haven't looked through your Album. My oversight... There's some great wiring 'Stickies' on this Site, but the bottom line is to confirm w/the prior Owner/Manufacturer where the wiring is, and assess the accessibility of it.
4 prong flat Connectors with 4 lead flat wire is, arguably, the Industry 'standard', so you can duplicate that on any Connector added.
Trailer Wiring
More Trailer Wiring
For what your Eye can't see, one nondestructive trick is to use ye good ole straight Pin and Digital Voltmeter [DVM]. With one DVM lead jumpered to Trailer/Wiring Ground, stick the Pin through each wire of interest while checking for Tail/License Plate light; Left Turn/Stop; and Right Turn/Stop functions. As my dear ole Mom used to say, 'to assume can make an a** out of 'u' and 'me'. After confirming the wiring you have - no other wiring protocol matters - you can parallel that wiring to the rear Connector configuration you like. Most DVMs won't jump the reading clear up to +12 VDC on a turn signal voltage. They'll show ~1/2 that voltage.
The Receiver pix references were posted as idea stimulators. I thought the fellow's rear Receiver idea to provide Camper Steps and hold his Generator might be something worth copying w/modification some day. I've seen plenty of Popups carrying everything from multiple Bikes to Canoes to A/C units on Aftermarket Roof Racks, so transferring weight to the side panels - as you've shown above - works, OEM design permitting...