I have not had any trouble draining the black tank either, and we only have the 2" cam lock coupler not the 3" hose most rv's use.
What we have found is in another application, a railroad car used by the public it did clog once in 10 years of use with toilet paper. So toilet paper meant for rv black tanks, or septic systems does help with that. Regular tiolet paper is a bit to strong and does not fall apart enough if enough of is in there. This car does also use the 2" cam lock. Mainly because the potti potty company uses that size fitting and hose.