You can improve the converter to battery amps flow by reducing the R of either the pos path or the neg path or both paths by any amount. Each path can have different Rs. It is the total R of both paths that counts.
You might still find if you improve the pos path that it doesn't help much since it already has fat wire. Assuming the neg path is mostly frame, any big R on that will be connections that need cleaning as was mentioned.
You can measure the R along a path that is carrying a known current with your voltmeter. Put one lead at each end of the wire. You can do this by adding speaker wire to the leads, even 40 feet of speaker wire will still give you proper voltage readings.
The voltage on the meter will be the voltage between each end of the wire and you have the current, so you get the R. You can do this along the path between sections and find where there is a big R in the path, such as across a fuse holder.
With the long speaker wire leads you can run this test from the converter to outside the rig to the batteries, by- passing the whole frame to see if the neg path is your culprit before rewiring the pos path on speculation. Measure both paths first and then see what to do.
