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We have only one or two things plugged into the 110 volt outlet. How would you suggest in checking the backup batteries? And do you charge them the same way as normal batteries are charged? Where are the backup batteries usually located? As long as the electrical cord is plugged into a 110 volt outlet it makes the humming noise with nothing plugged into any outlets.
I your rv there is a device called a converter. It is a battery charger and 12 volt power supply. If your unit is new enough it is a very smart 25 amp to 55 amp unit. Older units are a power supply and charger in the same box but seperate. The hum you hear is probably the cooling fan for the converter. We will assume you have a "newer" unit with the smart system. When you plug your trailer in it will evaluate the battery condition by going to "bulk charge" and putting a high voltage high amp charge into your batteries untill the amperage drops or a pre set amount of time passes. It then dropps to trickle charge. When it feels your batteries are charged it will drop to maintenace level. When it detects the batteries are discharged enough, usualy 50 to 75% charged it starts the cycle over. When you plug your unit in the batteries will need charging and that will cause the fan to come on, it is load and tempurature controled. The batteries in your unit could be under the step in the entry or in on of the exterior compartment doors or on the a frame of the trailer. In rare occaisions I have found the batteries in racks under the trailer. Proper RV batteries are not maintenece free and need there water topped up and conections cleaned. They will probably be in plastic boxes (55%) probability. The humming sound will not have anything to do withh house current load. Hope this helps.