Well, which is + and which is - depends on the batteries however there is an inexpensive solution, in fact there are two of them.
First ignore the short connecting wire, it's not part of this, go with the two that have multiple wires hooked to them
Now, the way I'd do it,,, I'd break out one of my voltmeters, You can get multimeters (and every RVer should own one) at many places, I got lucky at a dollar store once, Yup, 1 buck per meter. Harbor Freight is another resource. They will show you which is which (Red is + if the meter reads properly)
Option 2 is to use a light bulb coupled with your battery charger, here is how it works DO NOT PLUG IN THE CHARGER, now take any 12 volt light bulb and put the RED battery clamp on the bulb so it contacts the metal base but not the pin(s) on the bottom, they need to be exposed... If using a "Cartridge" shaped light (like the overhead light in many cars) hook the red clamp on one end so it contacts the wires. Hook the black lead from teh charger to either post, Tap the other bulb lead (the center "pin" or the other end) to the other post.. if the light lights.. You don't want to hook the red lead there, move the black to that post and try again, if the light does not light,,, Just for the fun of it switch the leads (This insures the bulb is good and everything is working the way I wish it to work)
Once you have the bulb lighting one way, but not the other, hook the black lead to the post that it's hooked to when the bulb does NOT light, and hook the red lead to the post you touched the light to.. Good to go, MARK THE POST some way so you can do it again easily
Method 3, wire tracing... One (At least one) of the big wires is likely very short and hooked to the chassis close to the battery box. This Should be the NEGATIVE (-) lead NOTE Should be