An inverter changes 12 volts dc to 120 volts ac. A converter changes 120 volts ac to 12 volts dc. The converter also serves as a battery charger for the trailer batteries.
I'd suggest that you join the Keystone owners forum and ask some specific questions there. For starters, find out what sort of converter you have. Some of the cheaper converters are basically "dumb" chargers, putting out about 13 volts dc all the time. New, "smart" converters monitor the batteries so that they aren't over-charged. You can also find out exactly where the converter should be located and what sort of tricks you need to do to get to it.
Should you want to add an inverter please do some research about what sort of capacity you need. If you have a typical rv refrigerator it will run on propane when 120 VAC isn't available (like when you are driving down the road). What do you want to run? The television? A computer? How much has to run at the same time?
Mercury Mountaineer towing mpg 181(both sold)
1993 Foretravel U300 40'