Your picture is way too big and annoying to many readers as it makes the page wider than the screen. To edit your pic size, click "edit" post. at the end of the html gibberish, put a space then "width=350" just to the left of the brackets that contain the letters img and a slash. I cannot use those symbols or the forum will reject my post as bad html...
Here is an image that has been sized to 300:
brackets here https://photouploads.com/images/EUrP.jpg width=300 brackets here
Black and white--black pos, white neg
Red and black-- red pos, black neg
Red and white--red pos , white neg
This is good knowledge to keep around. Write it down in several places and put one copy in your DC elcetrical area.
Sadly, most wires will not be paired up like this.
RV manufacturers can get quite creative with their wiring colors. The trailer brake/reverse/running lights will not even match the vehicle side colors. That is something not fun to learn the hard way...
All that I have seen do use white as the RV ground color, but this means nothing on a forum.
Because a car is almost always and only black for ground this should have been the pattern to follow, but obviously early RV manufacturers did not work on cars, ever.
Your best option is to get a multi-meter so you can test voltage. Ground one lead to the frame, anything metal that is likely grounded, and the other lead to what you think is a positive 12 volts . you cannot get it to work through paint, the contact will need to be scratched down to metal, or find a screw head for ground.
If you get it backwards the voltage will show a negative, -12.
If you think white is the ground wire then use that for the negative test lead, and touch all other wires to see if you get a 12 volt reading.
Poor wiring, poor conductivity might give lower voltage, like 8 volts, or 2. This means battery is dead, or that the wiring or ground or something along the way is not conducting the electricity very well at all. This means the wiring is wired right and all looks good, but the wire or something is bad making it not work.
Old trailer will get a lot of corrosion on contacts or where one wire is attached to another or to a block and that corrosion will inhibit the flow of power...