12 volt circuit at fuse panel only 3 volts

What is the condition of the battery? If it is working when connected, the converter is likely not the issue. Verify the voltage at the battery with a voltmeter/multimeter.

Also, check the ground connection between the negative battery terminal and the frame. That could be corroded. There should be two reverse polarity fuses. Check both of them. Did you have the battery disconnected? Are the battery connections tight with the correct polarity? Do you have access to another battery to set in temporarily to check (such as a 12V lawnmower or a car battery? 

The reverse polarity fuses are there to protect the converter if you connect the battery backwards, even for a moment. If one or both are blown, then somebody reversed the positive and negative connections at the battery. If they are intact, then the battery was connected properly. The second positive wire is likely from your solar controller. I am afraid I can't do much else from here. You may need to make an appointment with your dealer to get it checked out or find someone local to you who can do some diagnostics there. We are too far away for me to come and take a look at it.

+1  !!!  I had that exact thing happen to me about 5 years ago.  Battery tested out at 12.72v (100% fully charged.  Fuses and everything else checked 'good.'  Even cleaned the battery terminals and connections.  Still had a problem with the lights barely coming on, as well as the 12v water pump was dismal at best.  Did as TT wisely suggests above and checked the ground from the battery to the frame, AHA, corrosion city!  Cleaned that up, put on a new end piece, as well as dielectric grease, no problems since. 

Thanks for bringing that up Tars!

My neighbor suggested a tap into the ground wire then a connection to the frame in a second location.