If they are 12VDC batteries I would first make sure they are installed to the correct battery terminals. Find the word "NEG" or SYMBOL '-' printed on the battery case and make sure these all go to the trailer FRAME GROUND.
If RV trailer color codes were used this should be WHITE COLOR NEGATIVE cables paired with BLACK COLOR POSITIVE cables...
Sometimes the AUTOMOTIVE COLOR CABLES are used where RED is POSITIVE and BLACK is NEGATIVE.
Certainly easy to get these mixed up so I always look for the markings on the battery and find the FRAME GROUND CABLES which are always grounded very near the battery locations..
If they have been REVERSED than you will need to locate some blown fuses. One near the battery battery bank and two more on the 12VDC Distribution Panel labeled REVERSE POLARITY. Be sure to correct the battery terminals FIRST before changing fuses otherwise they will immediately blow again.
All of this info is if the BATTERIES ARE ALL 12VDC types.
Roy Ken
