Hi Tomnbeth,
If the handle pulls more easily than normal, it's possible the cable came loose from the valve's plunger. If you drop the coroplast, at least in the corner closest, or cut a 3-sided rectangle to gain access (front edge intact), you can check the cable connection to the plunger. If the plunger isn't moving, you could perhaps help it with a pair of pliers.
But it's much more likely you just have a difficult blockage. If you have an external twist-on gate valve for the sewer outlet, and a clear backflush adapter, you can force a water buildup that will clear the blockage. If you have only the backflush adapter hooked up to the sewer outlet, you might try back flushing while holding the sewer hose high overhead. The water pressure it takes to fill the lifted hose may be enough to dislodge a blockage. Take care not to overfill the black tank.
And be prepared for when the blockage breaks free. Unless your wife is shooting video.