Those older converters where well known for boiling out the battery fluids over time and if left that way the battery shorts out and has to be replaced.
A good multi-voltage smart mode charger will go along way keeping your battery very happy... I would not replace the converter/charger unit with another WFCO unit. Smart mode type Progressive Dynamics and IOTA are popular converter/charger units.
Yes you can use a portable battery charger in its place but keep in mind the cheap battery chargers were not designed to be used in a continuous mode. The good converter/chargers also have great built-in DC REGULATION circuitry that mimics a battery very well.
I carry a Black and Decker VEC1093DBD 40AMP Smart Mode Charger for a back-up in my tool box...
Also keep in mind if your older trailer has electric brakes the trailer battery is also used for the required EMERGENCY BREAK AWAY setup for your trailer in the event the trailer becomes disconnected while being towed. This is a DOT SAFETY REGULATION thing. Not a good idea to travel with a dead trailer battery.
Roy Ken