IF you haven't already, I suggest you read the manual. It tells you how to hook it up. Two of the wires from the trailer are the 12V
positive and negative cables to the batteries. Others are probably the shore power input and AC distribution. Since, to my knowledge, an inverter was not standard with trailers of your vintage how the new inverter and charger hook up will depend on how the original system was wired.
That said, with a separate inverter and charger, you are missing a function of the original inverter/charger, the transfer switch that powers some, if not all, of the trailer AC loads from either the inverter or shore power, whichever is available.
Further, it sounds like your previous inverter was intended to be wired into the trailer's 120V AC power distribution and this inverter has only the 15 amp receptacles. This may or may not be compatible with your trailer's wiring setup and the loads presented to your previous inverter.
I'd suggest you obtain the services of a local RV-knowledgeable electrician to understand how your trailer was originally wired and how to wire up your new equipment.