It may be required to trace / debug the wiring.
One thing that jumps out is the indicator lamps. On my system those become active when the head system has DC power, illuminating the appropriate level lamps and the POWER lamp, but your image shows the breaker ON and no lamps illuminated, including no power.
There is a secondary operation to manually select the discharge. Your panel has two different secondary operations (TREAT & DISCH, plus TREAT & HOLD), probably appropriate for the retired system.
The sanitation system MUST have a lockout to ensure that the system cannot be discharged w/o removing the lockout. It is common to use a padlock on the seacock, or a nylon wire tie, to secure the handle in the shut position. My practice is to close the seacock, then remove and stow the handle.
That system appears to have no possibility to comply with that requirement. It is possible / likely that an inspection would flag / fail for that condition.
An "easy" solution would be to retire usage of that above waterline discharge through hull and to instead properly install one below the waterline, so it may be disabled except when appropriate. Mine has a switch to prevent discharge, except when the seacock is open. Mine has a seacock one size larger, using a reducing fitting at the threads, to make a larger opening for the waste.