Could be a bad ground at the frame they have tendency to rust. Good advice also to check ALL of your power connections, temp swings and road vibrations will loosen things up.
If you are looking at replacing the good ol' WFCO converter, consider the
PD4600 from Best Converter. You get the charge wizard and same features as the 9200, and these are designed to be a slide in replacement for WFCO, Magnatek, and other units so you don't have to change the whole power panel. I put one in a few years ago and it does a much better job charging the batteries and keeping them maintained.
Depending on how far the batteries are from the converter, consider upgrading the (+) wire to heavier gauge to improve charging efficiency.