IdaSHO has it right -- If you want to charge a "house" battery off the truck, you need BIG cables to keep the voltage drop to a minimum or the battery won't charge well at all. For comparison to his 0.12v of drop from the truck to the camper, when I was trying to charge off the factory trailer wiring, I never saw more than 13v at the house battery, even when pretty much fully charged and not pulling any amps! That's ~1.5 v of drop at zero current, and more like 2.5v with even just 10a of charge current.
Best mod I made was an 85W solar panel and charge controller. Now I don't plug in the camper to the truck unless I want to run the fridge on 12v while cruising down the road. That said, I am still powering the fridge through the factory trailer wiring, and I'm only getting ~12V at the fridge when it's in 12v mode. (about 10A draw)
If I wanted the fridge to work better when in 12v, I would run some better wiring, but it stays cool enough that I don't worry about it.
I guess what I'm saying is that if you're going to the trouble of "properly" wiring to charge the house battery, it would be my opinion that you might consider a 100w solar panel and charge controller instead, so you don't have to deal with plugging/unplugging it when you load and unload the camper. (I do that a lot, so that's perhaps not a concern for you...)
Best luck!!
Chris