The short and not helpful answer is a steady high amp draw through the system heated up and melted the wiring as you drove. The only way I know to solve this puzzle would be to first determine what the was pulling from the batteries. It does strike me your fuse is not protecting your setup, but you already know that. Increasing the ampacity of the wiring may be helpful or decreasing fuse size. However, I am curious what your rig might have to generate such a high amp draw just going down the road.
I was wondering if you have an inverter that was left on for example feeding an appliance. I have seen high inverter draw blow the top out of batteries and I have seen wires melted into bundles with draws that generated steady heat, but were not high enough to blow fuses.
As you said, your systems worked fine for a number of years. What changed? I think this is a really interesting puzzle! Not being sarcastic.
Steve and Andra
2012 F350 6.2 gasser SRW LB
Fab Fours front and rear in case we run into a rhino
2019 Northstar Laredo SC