The extra wire may just be looped in and out to the next thing in line or do they go to separate bits on the light fitting? Can you determine Positive & Negative? Normally LED just won't work if wrong polarity, if so just swap em round.
thats really weird i did mine last year on a 1999 eldiss and its just two wires ,i think that possibly your two green wires form a loop system .my advise would be before fitting ,join the two greens and then add a metre long test wire to the two wires brown /green ,run them to your new light fitting and test it all works .
the only reason i could think of for a extra wire is if your old light was switched on inside the caravan and also by a switch on the light fitting