Which wire to positive on battery

It has been over a year since we got the camper out and I don't remember which wires go to positive and negative. I've got one battery and one red wire and two white wires. I know color doesn't mean much but I thought that I would mention it. The red wire is thicker than the other two.

I vaguely thought I remembered the red to positive and the other two to negative. But, when I hook it up like this the lights are dim and do not charge when I plugged into 110. Everything works except the lights and furnace. My converter is only a couple years old. Since I wasn't very sure about the wire connections, I'm thinking it had to be that I don't have them on correctly.

I've tried followiing the wires but it is very difficult as they seem to disappear under the floor. Like a car, would the negative actually go the frame?

Is there a way to figure this out using a multimeter?

Thanks in advance for any help!