When the trailer is connected to shore power, the converter supplies 12 volt
DC power to both run the 12 volt
items in the trailer and a charging current to the batteries.
The problem, as mentioned above, is the current supplied to charge the battery is not regulated very well by the less expensive stock converter.
This can lead to overcharging of the batteries and cause them to boil dry and ruin.
This issue can be manage several different ways:
Some folks choose to replace the factory installed converter with one of several on the market that do have better battery charging circuitry.
Some simply "manually" manage the situation by using the the battery disconnect switch.
This is the switch in the trailer that says "STORE" and "USE". Simply place the switch in the store position most of the time. This will disconnect the batteries from the converter and thus prevents the over charge.
But, remember to place the switch in "use" for 6-8 hours once a week to keep them charged. And, again place the switch in "use" when you have to unplug to move or travel.
The red led will stay lit in the "store" position when the trailer is plugged into shore power.