Been there, done that too. My converter seemed to be putting out great, but at the batteries.. nope.
So.. to troubleshoot this problem, take your jumper cables from your battery, and run to the 12 volt panel. Take short leads off your jumper cables, first to the positive and then to the negative..
Most times, just doing this will determine what is wrong with your system, and then you can determine what is really wrong. Changing out a converter usually results in renewing those positive and negative leads.. so in many cases, that is what you are told you really need, but usually it's just a bad connection somewhere.