I can't say for sure where it is, but may be able to help you find it. With the unit plugged in to shore power turn everything off that makes any noise. If the converter is working at all you should hear the hum of the cooling fan. Follow your ears.
Converters tend to be mounted low, usually at floor level, and will be in a ventilated space, probably behind a louvered panel or grill. In our Axis this is in the space under the fridge.
Thor uses a WFCO converter in many of their coaches. An internet search suggests the WFCO units have some known issues. One being failure to drop into "bulk" mode when needed, which could lead to the problem you describe.