Here is the wire, it is nice wire, with a huge temperature tolerance range and the convenience of being properly trailer color coded for easy referencing. https://www.ebay.com/itm/Trailer-Light-C...2749.l2649
It's interesting that you mention the ability to charge from the vehicle. Shortly after I got my suv a buddy and I ran 8awg power line all the way from the battery to the back. I used a 30 amp marine grade surface mount circuit breaker, and a battery charging and isolation solenoid , there is a fuse tap running off the main ecu fuse to the solenoid so that it only connects when the vehicle is turned on so there is no chance of draw on the main vehicle battery if it isn't running. I used anderson SB50 heavy power connectors and weatherproof boots at the rear of the vehicle and on the trailer. I am currently still working on my battery box and external outlets for the trailer so have not run the 8awg line in the trailer yet, but I used 3/4" flexible pvc pipe inside the tongue and the power lines slide right in there with the trailer wire no problem.
I did purchase new Spring Seats, will be disassembling tomorrow and my welder is putting them on friday evening.