Remember that there are pipes connecting all those tanks together in the front. If the trailer is not level then when you drain the black tank some of that goes up each of those other lines.
So, dump the black first, then the middle and front grey tanks. When you pull the grey tank handles you'll get a bit of black sewage as the lines are 'flushed' by the grey water. On mine the middle grey tank pipes can hold a gallon or more depending on how level the rig is.
The rear tank is completely separate from the front ones.
And, yes, when I got mine I did the food color trick to make sure which handles drained which tank.
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2017 Eagle HT 29.5BHOK (sold)
2017 Ford Powerstroke 6.7, Crew, 4x4 (sold)
2018 Toyota Highlander
Maggie, Old English Sheepdog
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