I grew up in South Jersey, spent most of my life there, and I've never heard of "jawn". I don't doubt that it exists, but it certainly seems overstated. I've spent some time in Philly proper, Manayunk, working class neighborhoods, college neighborhoods, and with large groups of Philly transplants in New York. Maybe it was said around me and just got lost in conversation, but I have no recollection of it at all, so I wonder how ubiquitous it really is.