I did exactly that on my rig. Although I also added a 3,000 Watt inverter. Reason: I used to carry a chainsaw for cutting felled trees for firewood and then split them at the camp site. Just plain got tired of having to mix the oil/gas ratio, smell, leaks and not always starting on the first pull... or second or third...
Installed the inverter, 100 foot of heavy gauge extension cord (I'll never cut wood further than 100 feet from my vehicle) and a very nice 18" bar electric chainsaw.
Here's the link to my build:
Builds - R2M 2013 GX 460 Expo
The inverter is also great for many other uses, such as power failures. I can run the extension cord into my house, plug in my refrigerator, TV and several lights and wait out the power outage. I can also use microwave ovens, any time/place where electricity is needed and it's too far from a building or other structure with a receptacle. I love having it.
But onto your issues, I used 1 AWG wire and ran it straight to my second battery. I ran it along my rock sliders tucked way up high so it could not get crushed.
After I installed it I purchased a small unit that has (1) 12 VDC socket (cigarette lighter type), (2) high output USB plugs. It also has an On/OFf switch so it doesn't run down my second battery if I forget to unplug something. And it has a voltage meter to monitor how my battery is doing. I inserted this all in the panel door where the jack goes on the drivers side. You can see what I did in my build thread.
Just a heads up, the part about adding the 12 VDC jacks will be on the third page towards the end.