You may have either a defective tank valve or some debris trapped in the slot that the valve blade seats in when closed. For the former, it may be the cable is not traveling far enough to fully close the valve, or the valve body screws have loosened. To deal with that will require dropping part of the coroplast or making an opening to access the valve. For the latter, getting TP or other organic materials out of the slot might be possible by adding tank treatment chemical to the tank and then backflowing water up the sewer connection to open tank valve and letting it sit for a period of time, possibly dissolving or loosening it. Do not fill the tank, add just water enough to try to cover the lower portion of the valve opening and let the concentrated chemical try to do its job.