One of the black wires, and one of the white...
You can see the negative sign behind the empty post (2nd pic) - on the white wire side.
On our 88 Mallard we have 2 blacks going to the hot and 1 white to the negative due to the vehicle charges the battery too while driving. If you pop the door on the converter and look at the circuit board usually on the right it has "car style" 15 or 20 amp fuses there are 2 terminals that accommodate 8 gauge wire. In newer converters they will say NEG and the other POS. If you see and should see the black hooked to the POS (positive) and white to the NEG (negative) that is how the wires should hook to the battery. If reversed than hook up accordingly. The converter should have 2 - 20 to 30 amp fuses, polarity reverse fuses)next to each other that will blow if you hook it up wrong so you won't damage the converter. It's the same as house wiring; black is hot and white is common or ground, You can also Google your converter and the manufacturers' web site usually have installation or owners manuals to download the will give you wiring diagrams.
Greg & Diana
1988 Holiday Rambler Aluma-Lite XL 31' Banks Power system, & Tag Axle