Multiple shapes - converted into single lines ?

Hello, Working on a map, and I have ZIP code boundaries. where each ZIP has it own shape (fill and stroke). So multiple shapes are touching since their borders line up against one another. My goal is to just take the outline of the boundaries and create a single dashed outline. Problem, everywhere borders line up against each other the dashed borders loose there constancy in dash lines since its overlapping another boundary's dash line.

Now I have done this in the past and can't figure out what it was I did, so I know it possible. How can I make all those shapes and just create a single line where the borders match up?