The 55 amp rating is for the 12 volt
side of the converter.
The converter is a device that changes the 120 volt AC ( alternating current) to 12 volts DC ( direct current). The DC is powering the majority of the devices in your coach. Lights, furnace, ceiling fans, oven exhaust fan etc.
The converter charges the battery(s) in the coach along with powering the devices when plugged in to AC power.
A fully loaded converter rated at 55 amps on the DC side will require about 5.25 amps on the AC side when the losses thru the converter are counted.
The reason the coach has a 30 amp service is to provide power to the converter (5.25 amps); the Air Conditioner (12 to 15 amps); along with the 120 volt outlets in the coach; the electric heating element in the refer (200 watts or more). And perhaps an electric element in the water heater.
Thus the reason for the 30 amp/ 120 volt service at the power pole.