First of all, in an RV, the Disconnect Switch does not actually disconnect the 12-VDC line. It is a cheap low current switch that applies or removes 12-VDC to a higher current solenoid that can handle the current that might be going from your battery to all of your interior 12-Vdc appliances.
So, you might want to check that solenoid as being your intermittent problem.
As to your Hum? Converters typically do not make any noise(such as your Humming sound) unless they have a heavy load on them.
Check out the solenoid as being your intermittent problem and see if the solenoid takes care of this. Then see if the noise continues.