John,

Our system 2017 Discovery, allows us to hold the flush button and override the "full tank" issue for several flushes, I don't believe there is a master override as the results of overfilling the black tank would be a disaster you don't want to face. 

If you feel the tank is filling to fast, 2 things come to mind, 1st the sensor may be reporting wrong, having a through flushing of the black tank might help. 2nd the flow rate on the flush may be set too high. You can adjust the volume of water used to each side of the flush. We have ours set to the minimum needed to clear the toilet and we regularly camp a full week at the Nascar races without filling the tank.