When you are plugged into 120V shore power, measure the voltage across the battery terminals. If it reads 13.5 or so the converter is running. Slowly walking around your rig when everything is quiet, you may detect a small hum or a fan running which may give away the converter's location. If the Battery voltage is less than 12.5 or so then the converter is not likely functioning. Every manufacturer hides them in different places, so perhaps someone with a similar unit to yours will pipe up with a better answer. On some models, the 120V Circuit breaker and 12V fuse are located in the same panel (often brown coloured) and the converter is built-in behind that panel.
Have you got a cutoff switch used to disconnect the batteries while the unit is stored? Is it "ON"??
If it is on, you will have to locate the associated big relay and possibly figure out why it has not operated and/or latched. Using a voltmeter is highly recommended...
HTH at least a bit...