[Carl n Susan]

The black stuff is called coroplast and is attached to the frame using self-tapping screws. Above it a thin plastic like insulating blanket whose name escapes me (reflex?). Remove the screws in the coroplast and you will be able to drop it down. The insulation just lays on the coroplast and is not attached. Undo the brackets for the gas line which should provide enough flex to slide the coroplast out from under it.

Besides the drain lines, you will find: water lines, hydraulic lines, 12V power lines, 120V electrical lines, sawdust, wood scraps and God knows what else that might have been left there.

I doubt you have a leak in one of the ABS sewer lines. Dollars to donuts you have a crack in one of your tanks. Lots of members have had them. Do a Search here for "tank repair" or such to find lots of discussions. Often the only viable solution is to replace the tank.