That thing in the photo is a converter (also called a Univolt in the owners manual). It converts 120 volt alternating current to 12 volt
direct current, to charge the battery and power the 12v
lights, furnace, etc.
Unless it has been added by someone, your 1980 trailer does not have an inverter. Inverters convert 12v
Humming is normal. More important is to measure the voltage output to make sure it is within the range stated on the data plate that is on the cover. That type of converter can cause damage to the battery if the battery is not kept full of electrolyte. A high non varying voltage rate of charging can boil/evaporate the water out of the battery fairly quick. Some people do replace the Univolt with a modern mult-charge stage converter. (I did in both of my trailers after the old univolt failed)
If you choose to replace the converter with something more modern, I suggest you contact Randy at Best Converter (bestconverter.com). Randy is a member of this forum, who has offered guidance to me and many others.