Using the frame for grounds cuts the possible voltage drop by running the ground all the way to the battery.
SOME devices either require a separate negative lead run to the battery or just work better if that's done.
Examples would be Inverters, items like DC-DC chargers like the Renogy unit used for charging Lithium batteries from TV alternators.
If using the frame/chassis for ground/negative connections, don't forget to give them proper attention. Just like battery cables they require periodic cleaning to remove corrosion that increases resistance at the connecting point. These connections are the most overlooked items in an RV 12 volt system and are often the most common cause of problems because of this neglect.
"A wise man can change his mind. A fool never will."
"Sometimes you're the dog, sometimes you're the tree"
2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan