I did read through that, nice work.
Mine is a 50amp rig. I was hoping it was pre-wired, but it is not. My second unit is only 7,100 BTU. It was a NIB unit (Coleman Mach) off of Craigslist I got cheap. My W&P was built with (1) 13,500 BTU unit, so this addition should bring it up to 20,000 BTU.
I will be fishing wire similar to what you had to do, but will be going to the breaker box under the fridge. There are (2) open slots for additional breakers there.
I plan to kill two birds with one stone and add an outlet in my front storage compartment, to run my small air compressor with. So I will be running (2) wires to the breaker box.
Just waiting on the new seal kit to arrive from Amazon, before I tear into it.