If you have any doubt, you can figure out which is which.
First make sure the wires are prevented from touching the frame, and then plug in your camper so your power converter is powered. Using a digital multimeter, check the voltage across the two wires. You will get a reading around 13 volts, either +13ish volts or -13ish volts depending on which way your meter leads are connected. If you get +13 volts then your leads are correct and your red lead is connected to the wire that connects to the (+) terminal of the battery.
If you get -13 volts then your leads are reversed and your red lead is connected to the wire that connects to the (-) terminal of the battery. If you reverse the mulitmeter leads, the meter reading should be +13 volts and the wire now attached to the red meter lead will be the one that needs to go to the (+) terminal of the battery.
Now here is a little trick I did to prevent reversed connecting in the future once I figure out the polarity of the wires. Most deep cycle batteries have threaded studs on the + and - battery posts for either wing nuts or plain nuts. USUALLY, one is bigger than the other. IIRC the (+) post stud is typically 3/8" and the (-) post stud is 5/16". What I do is get out my terminal crimper and put a 3/8" ring terminal on the (+) wire and a 5/16" ring terminal on the (-) wire. The wires are then "coded" by size and will only go on one way since a 5/16" ring terminal will not fit a 3/8" post.
FAA A&P mechanic, ex-RV dealer parts dept
Retired B-52 Gunner/Flight Examiner, B-52 Crew Chief
2016 Jayco Eagle HT 29.5 FBDS w/ 15,000 BTU AC, 50 amp service, upgraded counter top, thermal pane windows, elec. stab jacks, outside grill, backup camera and MORryde pin box.
2011 GMC 2500HD DURAMAX/ ALLISON