12 volt items not working - new power converter and nothing

by exterior lights, do you mean the TT lights, brake light , turn signal etc..
those are connected to the Harness that connects to the tow vehicle
they are not part of the RV section, are you plugged into shore power ?
check the voltage at the ouput terminals of the new wfco converter
this sounds like a missing or bad connection someplace
either a main fuse or the main ground/negative wiring