It is OK if the batterys are disconnected, it may not be OK if they are not disconnected.
That battery minder puts out approx 18 amp hours in a 24 hour day. If the trailer has a parasitic draw higher than 18 amp hours in 24 hours then your batterys are in trouble.
Best to disconnect the batterys from the trailer.
2007 Surveyor SV230 - 200 Watts Solar/MPPT Controller - 230 AH Battery Bank - 600 watt PSW Inverter - (2) 2000 watt Inverter Generators - LED Lighting - Boon Docking 99% of the time.
2009 F150 - 5.4 Litre - 6 speed - Tow Package