Originally Posted by Koppgd131
I was originally told by the dealer to leave the valves open all the time. When I complained to the service manager he said do not leave them open. Keep them closed and empty when 3/4 full. I've used the deodorant dissolving disks. I've also filled the tank 3 times with clean water and flushed them.
OK so now that we know you kept the valve open for a period of time we can surmise you have at least one (or several) hardened poop pyramids on the bottom of your black tank.
It's going to take more than filling the tank a few times and dumping to break these up.
You'll likely need to fill the black tank and add an enzyme type chemical (Rid-X RV or Happy Camper, etc.) and leave it sit for a few days or a week and then dump and see what comes out. You may even have to do this twice. That stuff gets like concrete! Driving around to slosh things up when doing this would help too. (if possible)
Use a clear extension or ell to watch for debris when you dump.
In the future, after dumping, always put back in several gallons (like maybe 3-5) of water, along with your favorite chemical into the black tank and it will be ready to use on your next trip.