Question regarding DC to DC Chargers

Son of Norway wrote:
I am installing a Renogy 20-amp DC to DC charger. My question is regarding the negative cable connections. The instructions say to connect the input negative cable to the chassis battery and the output negative cable to the house battery.

My chassis battery is a good 15 feet away from where I'm mounting the charger and it would take some tricky cabling to get it there. Both the chassis battery and the house batteries are grounded to the frame. The charger will be right next to a negative post on the frame. What difference would it make if I just connected both of the negative cables of the charger to the same post? Can this cause a problem?

I will contact Renogy next week with the question, but I am trying to get this install done this weekend, so I would appreciate any advice. Thank you.