You've got some digging around to do...
I wouldn't hook up the new converter/charger till you have all the gremlins chased away - you might damage it!
sounds like you have one of the POS cables or wires shorted to GROUND...
Get a digital multi-meter so you can trace each leg of the
12 volt wiring, starting at the batteries....
Check the wiring around the
12 volt fuse panel carefully - if any wires look questionable, be sure to clean them up and check for proper continuity, etc...
do you have any circuit breakers on your
12 volt panel? check them carefully for proper operation - they're internal contacts can wear out over time...
it might be easy for a wire near the 12 volt panel to be touching the alum skin, causing the 'short' - again check each leg of the 12 volt path from battery to 12 panel, then to the connections on the new converter/charger (are they hooked up properly?)...
I've seen some nasty looking wiring jobs done by PO's - if your 12 volt wiring is exposed, frayed, using 'wire nuts', etc., you'll save yourself many problems down the road if you take the time to sort it all out now, in a neat orderly manner, so it's easy to trace the circuits...
good luck
Ray