I also used the Camerasource kit on my 2020 3500HD and slide in camper.
There are several suppliers on Amazon for the Coax cable. I purchased a short section to extend the connection point to behind the license plate as not to wear out the factory connector next to the spare tire. Male to Female and the camera kit can plug into that extension when installing the camper or tailgate.
Good luck.
I
Check Torklift site for 2021actual part numbers but I found the install very easy on my 3500 HD 2020 7 Ft box.
Rear with OEM hitch . Safety cable hooks to a hole versus frame bracket and I added a SST link .
Good luck
My White 3500 HD LT True North Edition Z71 6.6 Gas with BLACK running boards . Round was my choice as flat ones tend to have the snow stick much more than flat ones.
I added a AVS Auto Ventshade 622187 Aeroskin Flush Mount Chrome Hood Protector for 2020-2021 Chevrolet Silverado 2500/3500.
Chrome on my white 3500 HD looks factory