clearly the charger isn't charging.
the simplest approach is to use a traditional charger for these few days,
whenever shore power is available.
IF rv battery use is limited to the fridge electronics and cooling is done with lpgas this will get ya through the trip.
while parked/plugged in the furnace/water pump and lights will still require 12v
but direct REcharge as needed will work.
the trailer brakes SHOULD function via the 7pin, but please be sure of that.
it's never fun to make major repairs while in route or on a short trip...
especially when the goal of the trip is a few days of family fun.
shipping parts overnight is crazy expensive and u might be hurried into picking the wrong (expensive) replacement.
you will be passing by several BIG camping worlds, so it is possible to stop and buy the gear.
but unless you wanna replace the uni' with another uni', the process of install/upgrade will use a full day (or more)
sometimes it is necessary to modify plans to accomplish the main goals.
ultimately the memories should be about disney and kids.
so if it's necessary to ditch the fridge food or use ice chests or buy a small 12v
cooler (plugged into the tow vehicle)...
i'd simply wait on replacing/upgrading the trailer charge system,
till the fun part is over and the rig is back in dry dock.