ok, I can add a bit to what kmcintyre said.
I used 2 AWG cable wiring my Hawk shell to my 2500 HD. Both +ve and -ve leads are over cable, meaning I am not using the truck/chassis for ground/return wiring.
From the truck battery, the -ve cable goes to towards the back with out any breakers/fuses. The +ve cable runs 8" to a 100A breaker
https://www.amazon.ca/Blue-Sea-Systems-Surface-Circuit/dp/B0051P01BW/ref=sr_1_3?ie=UTF8&qid=1478829489&sr=8-3&keywords=100a+breaker
I mounted to the plastic bits that support the fuse panel on next the battery on the driver's side of the engine bay. From there the two cables run over the fender liner/under the brake booster and parallel to a bundle of wires already in the harness there. I tuck them along that wire harness all the way back to the end of the truck cab, and then punched two 1/2" holes in the truck bed just over the pipe/hose from the fuel filler cap. Those holes are lined with plastic chassis guides to prevent chaffing. Everything is zip tied and tucked away from moving bits along the way.
One the cables come through the truck bed, I left about 12" and then cut them and attached anderson power pole connectors (actually, the connectors need to be put on first, time wise. This is a logical flow description, not timeline!)
I removed the wires from the passenger side , which is where my shell originally had some dinky 10g wire run from, when I gutted the shell last fall. So now I made two new holes on the driver's side and ran the cables from that side over the water tank area to the passenger side next to the water tank, which is where my battery/electric box is. I left about 18' of slack on the outside of the camper and attached the anderson power pole connectors there. You need that much so you can (dis)connect the two without having to try and manage that through a turnbuckle door. Been there... don't like doing it that way!
At the battery box, I have another of those 100A breakers on the +ve cable and from there it connects to a fancy battery separator, much better than the typical factory installed ones - the blue sea ML-7622 ACR.
http://assets.bluesea.com/files/resources/instructions/990170140.pdf
The ground wire does NOT go directly to the camper battery either, but to my battery monitor's shunt. I use a Victron,
http://shop.pkys.com/Victron-Energy-BMV-702-Precision-Battery-Monitor_p_2807.html
but the Trimetric would be wired the same way. The other end of the shunt does eventually go to the battery -ve terminal.
Does that help?