So with no positive wire on the lead acid battery, it is not being used at all. I assume that one of the wires on the negative terminal go to the chassis ground. And i assume that the positive wires on the agm batteries go to various connections for the trailer. My guess is that the former owner was not able to remove the lead acid battery for some reason. Without being there to really look around, this is the only conclusion i can come to.
2012 F350 6.7 L dually, 2013 3800RE with 6 pt leveling, Sumitomo 17.5" load range h tires, Samsung 18 cu ft residential fridge, 8k Morryde I.S. with disc brakes. Full timing since 2012.