Power Supplies for Air Streams
Not all but many are merely an AC transformer that have voltage doubling and a diode arrangement to supply 12 volts to the trailer and charge the batteries.
A 55 Amp unit is capable of delivering up to 55 Amps but does not push 55 Amps through your battery.
It is like having a gallon bucket and taking out only a once at a time or what you need. one ounce , two , or more, but capable of 55 amps if needed.
Your battery has an internal resistance, and this is low when the battery is used up and the current from the charger will supply enough voltage and current to charge the battery. As the battery is charged the internal voltage will rise in the battery and the resistance increases, and limits the charging current. Once fully charged the voltage in the battery is the same as the charger and little if no more current is delivered to the battery.. An automatic limiting system.