That glue (black goop) is butyl tape. It comes in a roll and is squashed down between the canvas, furring strips and van roof. The furring strips are thin strips of wood (think of a wooden ruler) that are run along the edges of your PH top holding the canvas down to the metal with screws. I'm thinking you hav lost some screws and possibly have some rot in the furring strips if you have that much of a separation.
You can pull the upholstery back out of the way pretty easily to inspect all the way around your top. It is just spray glued down, easy to stick back in place.