Originally Posted by Ford Idaho
This will not be a quick easy job so a little patience will go a long ways.
Water is your black tanks best friend so close the valve and add as much as you can and add a waste digesting treatment and let it sit 24 hours.
Drain the tank and then use one of the wands poste above with the tank valve open because it is going to take a lot of water.
If the clog is in the tank, your only hope is using a dissolving treatment like Unique's RV-Digest-It or a healthy dose of RID-X and a 72-96 hour "soak"
Be sure to follow directions and add enough water to fill the black tank (without overflow. There is no fast fix that does not involve a tank replacement.
As you discovered (the hard way), do not use flushables or dump with less than 3/4 full of water.
In the case of RVs; "down" is never "gone"
You most likely buggered your tank level indicators as well. There are several good threads on black tank maintenance. While your tank soaks I suggest you read up on it.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW