Strange Electric question...

If you want to verify which wire has +12 volts, ground your meter black probe to either the battery negative post or the chassis of the RV and then read the wires for the unit. Only one should show 12 volts. That will be the one that is positive regardless of color. If you really want to confirm once you have established which wire reads +12 volts, switch your meter to the continuity setting with your black lead still grounded and check the other wire that didn't have +12 volts. It should show continuity since it should be the ground. It certainly looks like the wiring doesn't match the colors. Once you confirm the wires are backwards coming from the source, you have a couple of options. One, you can reverse the wires before they go into the Winegard so it is seeing the correct voltage (easier, but not preferred as someone else at some point is going to be trying to figure out why it is wired the way it is). Or, two, you can backtrace the wires from where you are testing to find where they got reversed (preferred, but can be an extremely frustrating endeavor) If you have some type of switch inside to turn the unit on/off, I would look there first.