No info on your rig, so it's hard to diagnose. On my Cedar Creek, the black tank rinse line goes up underneath the bathroom sink, where there's a vacuum breaker valve, The line then goes down to the side of the black tank. Most likely, this breaker valve is stuck closed or obstructed. Mine's clear plastic so you can see if it's working. Find yours and you'll probably solve your problem.
First you have to manually close the solenoid valve that controls the slides. The valves are cylindrical and should be located to the left of the motor (2 of them together or 4 if you have the Level-up)) with hydraulic hoses going to them. They should be marked; Slides, Landing Gear and ODS, DS for Level-Up. You need to use a 1/8 inch allen wrench to manually override the solenoid. Stick the wrench in the end of the valve and turn fully clockwise to override it. Then, use the drill to turn the motor - clockwise to extend slideouts and counterclockwise to retract. This procedure, with pictures is on the LCI web site.
Cedar Creek does not make the fiberglass panels they use - they purchase them from the same manufacturers as other trailer makers using the same wall construction techniques. Kevin is not a structural engineer and his statements sometimes are somewhat misleading or oversimplified (I have dealt with him before).
All fiberglass will delaminate if water gets in and weakens the adhesive. You have a leak somewhere which needs to be addressed. Your expectations regarding seam integrity appear to be a little too high for any vehicle that travels down the road and flexes.