The AM antenna in most radios is a coil of wire around a ferrite bar, in the back of the radio. The oems do not do a great job of radio design. If you like AM I would recommend upgrading to an automotive grade radio. Kenwood makes some good ones that still have DVD/Blu Ray players and Bluetooth in them.
The AM radio antenna in most all radios today are only built into the radios them selves. The one on the roof is for FM only. Approximately 30 inch long wave length. Heck even the radio in my Prius is the same way. Removing it to go through a car wash and AM still works just fine. Not so the FM.
the am radio that is in most of the cheap dvd systems in most rv's is very bad even with a outside antenna the fm reception is not great either. the converter and some led lighting make am reception worse. you can try a standard old style car radio antenna on it or a antenna listed as a rv radio roof antenna but don't expect much.
buy an indoor antenna to connect to the radio if it has an external antenna connection point. Otherwise try adding wire to the end of the 6 foot wire until you find a length that gives you the best signal.