Black holding tank won't drain past 2/3

Ritkei wrote:
After 4 days of camping we have a full holding tank. I open the black valve and hear a rush of fluids for about 1 min, then nothing. Check the tank level inside and it shows 2/3 full. After searching I found a post that suggested filling all grey tanks and pull all the valves at same time, causing enough of a vacuum to get the holding tank to drain. This worked once and now I'm stuck at 2/3 full again. Never seems to drain below 2/3 full. Any ideas or tips tricks?

Everybody has commented on the sensors.........but your one statement (highlighted) makes me think you have a blockage occurring when dumping.

This could be the result of the one of the cut-out disc (installer cuts a hole in tank for drain line in/vent line/drain line out) was left in tank. It will float around and then when draining it gets sucked down to drain outlet and blocks tank drain.
This is IF the tank is 'really' only partially draining.

Cut-out discs left behind are a pain in rear-end to remove.
Drain valve needs to be removed and then disc grabbed hold of and pulled out.