Negative/Ground wire

You can add a wire running from battery neg to the neg battery lug on the DC fuse panel. This will make a neg path in parallel with the existing path via the frame. That will lower the R of the neg path and thus for the whole circuit. You will likely see more amps to the battery from the converter when you are charging the battery bank.

Only thing is to make sure the new wire path is fat enough gauge to handle all the amps in case one day you lose the original path (as you are worried about) and it has to carry all the weight.