could you pass along the converter model number... Might give someone on here a clue...
Your frame ground and DC NEGATIVE cable on the converter should not be switched. Only the battery connection should be switched.
Also these KEY SWITCHes are not the best quality one to use. Im not sure what the contact rating are but have heard of them shorting together internally when switched under under load...
The BLUE SEA Marine quality switch contacts are rated at 350AMPS and have the make before break contacts which helps in reducing arcing when switching...
My battery setup has all of my NEGATIVE terminals going to a separate FRAME GROUND connections so using the negative path of the batteries didn't work out for me... My four position battery switches are located in the POSITIVE side of the batteries.
This is quick drawing of my battery switch layout which is only unique to my installation. Might give you some food for thought for your installation.
quick shot on my battery install on my popup
Roy Ken