For proper balancing, any charging source or loads should connect to the + on one battery and the - on the other. If there is only one ground wire, put all of the positive wires on the other battery.
There seems to be a bunch of positive wires already, making that difficult without a buss bar.
If the heavy pair of wires on the lower battery are for the converter/frame ground and the heavy pair of wires on the upper battery are the solar charger, I'd switch either the ground or the power heavy wires. The smaller ones probably don't matter much.
