Depending on your intended use of your trailer, I would forego the converter and go directly to a quality inverter/charger like a Magnum MMS-1012. Several reasons:
• even the best 'converters' use a single set of charging parameters. Since batteries do not have a 'one size fits all' set of charging voltage requirements, a converter is a compromise. inverter chargers in general, and Magnum in particular, can be fully programmed for the exact voltage charging profile for your specific battery type and can also be custom programmed for special requirements like the constant current/constant voltage profiles needed by the emerging lithium battery technology.
• all converters (with one exception) do not use temperature compensation in their charging regimen. Since the voltage requirements for all batteries change with the ambient temperature of the battery, lack of a temperature sensor and applicable software could possibly lead to overcharging in very warm conditions and under charging in cold temperatures
• converters have a rated amperage output, which might not be best for your particular battery bank's amp/hour capacity. The common industry formula for charging amperage of a battery bank is the C/5 rate, with C being the amp/hour capacity of your battery bank. Example: you have a 200 amp/hour battery bank. C/5 would be 200/5=40 amps. This is the ideal rate of charge to put into your batteries. Inverter chargers can be programmed to input a percentage of the charging section's output and can be set to exactly match the C/5 rate of your individual battery bank……converters can not.
• converters do not have an adjustment for absorption time, which is also related to the size of your battery bank. Inverter/chargers can again be programmed for this feature to exactly match the requirements of your battery bank.
Yes, they are more expensive than a common converter and they require a bit more technical expertise to install, but they will make your battery bank last far longer than a converter, and you also get the production of 120VAC from your batteries as a bonus!