This is my battery bank setup using frame grounds at the battery bank. My converter/charger negative battery cable also is grounded to the trailer frame. I too am using 4AWG good quality marine grade battery cable.
When my converter/charger is operating I see a good 52-54AMPS DC Current being put back into the battery bank which tells me the frame ground connections are good.
My PD9260C is only about 5-feet away from the battery bank main switch but the 4AWG cable run to the DC Power Distribution Panel is around 12-feet away which also has the negative ground battery cable connection going to the trailer frame ground.
I did use the automotive method of tapping a hole into the trailer frame and uses a 5/16-inch thread tap. Then the Battery cable has the proper ring terminal with a STAR WASHER between the ring terminal and the frame ground to bite into the metal when tightned down. I always only use ONE BATTERY CABLE per frame ground connection.
The TAP BIT and Automotive Bolt and STAR washer is from PEPBOYS...
I might add that my install using this method since 2009 shows no signs of corrosion at the connection points.
Roy Ken
