The coroplast, corrugated underbelly, is likely held in place with either self tapping screws or a nail type rivet. If screws, get a drill driver with the appropriate socket and remove one side to look into the cavity. If the nails, you may need to use a screwdriver and hammer to loosen then the claw hammer to pry them out. There may also be an insulating foil liner above the coroplast and possibly a tape holding the parts together. If the tanks are more mid ships, then you may find it helpful to cut the coroplast side to side and peel the coroplast either to the front or rear. Probably starting on the street side would be advised as that likely has more potential plumbing.
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