First off ... Welcome to the forum!!
There are two things you can do. Disconnect the batteries (preferably using a disconnect switch) or use your converter or a separate trickle charger at the trailer. You could also try solar, but I gather you are looking for an easier solution
I would leave them on the 5er and disconnect. They'll last months this way without any harm to the batteries if you disconnect after fully charging the batteries. Mine were at 89% SOC after two months storage last winter.
I prefer to disconnect rather than apply a float charge through the converter because I don't fully trust the WFCO converter in my trailer to not boil the batteries dry. If I were going to leave them charging all winter, I would use a dedicated trickle charger. If you use the 5ers converter, than check the water levels every week or so.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD