The two 12s are in parallel ok in that old photo. Your negs go on the neg of one battery and the pos wires go on the pos of the other battery. Your negs are on the top left so all the pos go on the top right with the batteries reversed like that.
Your reds should go to the same post on the one battery, not one to each.
I am not sure what the white wire is there, so you would need a meter to check if it is pos or neg.
Disconnect the DC wires into the converter and check the voltage across them for that 26v. It should be more like 12.7 with new batteries in parallel.
Did you put the new batteries in the same way, reversed, or if they are now lying the same way, but wired the same, that might get them in series and 26v.
