Shouldnt have to cut the coroplast. Remove the back wall of the basement/pass thru storage and you should have to both black and grey tank valves. If you pull the cable and it's pulls freely but nothing is coming out of the tank I'd say the set screw came loose, or was never tightened, on it.
2017 North Point 375BHFS
2015 Silverado 3500HD Crew Dually Dmax 4x4