It depends on the trailers built in converter and the external charger.. There are several things that can happen.
1: They combine their efforts resulting in a faster re-charge
1B: this faster recharge may or may not be faster than the battery can be safely charged (Too fast shortens battery life) NOTE MAY or MAY NOT be, You got to do the addition and measurements.
2: Converter sees charger output and says 'Ok, full up" and goes into float mode
3: Charger sees converter and goes into float mode.
Bottom line is: Try it and see
using a good Clamp on DC ammeter, or a good Shunt Ammeter. measure charging current with just the converter, with the charger and with both, Compare to battery size and figure it out.
