Tank flushing is NOT a necessity....nice but NOT needed.
Use enough water when flushing toilet (fill bowl full prior to flushing toilet then count 'one thousand one')
Dump black tank when at least 3/4 full
Add couple of FULL toilet bowls of water back into tank after dumping so tank bottom stays wet and sloshes around.
That is all that is needed to keep black tank from having issues.
No toilet wands (some have come apart ---heads/flex in tank)
No dragging hoses inside rig
No cutting holes in side of tank/rig for flush kit
No leaky vacuum breakers spewing water inside rig
As for bathroom sinks (and some showers) draining to black tank.
YES....that happens and there is NOTHING illegal, unsafe or unsanitary about it.
(Industry Inspector would already be aware and would have NO recourse)
For MANY years RVs only had one waste tank.....grey either drained onto ground, in a bucket OR co-mingled with black
Again..nothing illegal about grey/black same tank
Don't make RVng/Camping so complicated