tulegit,
It is likely that there isn't one way to do it for every converter as their are differences.
When I installed a new charger in my trailer, it was easy enough to separate the charger portion of the original WFCO converter from the fuse section of the converter by simply removing a fuse. If you are unable to look at the diagram of the converter (or reverse engineer it as I did), then try calling the manufacture.
While I don't know your converter, this technique "may" get you there. Assuming your fuses are in the same box as the converter is, try removing the largest of these fuses (it may be slightly separated from the others). Then plug in your converter (plug your RV into 120 volts) and turn everything on. If the battery doesn't start charging (with the original converter), yet all the other DC circuits work just fine, then there is a chance that you got the right one.
Once you do isolate the output of the converter from the RV's 12 volt electrical system, don't forget to disconnect the input of it from your RV's AC power system (120 volts AC). Of course, only do this when the RV is unplugged from shore power.
Steve