You'll need to open the junction box on the front of the frame and see what is connected where. The white wire from the battery is ground and is probably connected to the chassis close to the battery. The wire with the fuse is going to go to the converter. I would trace down the other black wire and see for sure where it goes. I doubt Jayco used a double wire to handle feed to the converter as it would have been easier to just run one heavier wire. The 30A fuse also makes me think that's not the case or else they would have protected the double wire run with a heavier fuse. If it's not connected as a ground, check the manual for your coverter and make sure there isn't two 12V inpust for the converter before you do anything with either black wire. I'm guessing that one black wire goes to the DC distribution for when you aren't plugged into shore power, and the other is the wire that the converter uses to charge the battery when you are plugged in, since the battery is diabled on shore power but still need to be maintained(charged)Also, I wouldn't up the size of the 30A fuse. it was sized that way for a reason. I also would not increase the size of the 30A fuse without calling Jayco. it was sized that way for a reason., probably because they used 10ga wire and it's only good for 30A. Just my guess.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner