You'll never get it all out of the tanks, black, or grey. Most likely what you saw was whatever was leftover in there that you didnt get out due to a low spot in the belly of the tank, trailer not being tipped enough, etc.. As you travel it down the road the leftover sloshes around and will collect at the mouth of the valve at the tank. You open the valve and, Surprise!
Me, I'll flush as best I can at the end of the year, take the trailer home and put a bucket under the the end of the sewer line and open the valves. As I winterize the water lines, all the water that comes out of the lines goes into the tanks and straight out into the bucket at the end of the line. Once I'm done I'll use another couple gallons of pink pop and dump them down the drains into the tanks until I see some of it going into the bucket(s). This way, again you'll never get all the "stuff" out of the tanks, anything left in the tanks will not freeze and cause damage to the valves. It also helps keep the valve seals lubricated and sealing properly.
A lot of people complain about how much it cost for the couple 2 or 3 gallons it takes to winterize their units. I have 5 tanks on mine, 2 black and 3 grey. 212 gallons total. 2 bathrooms, 2 kitchens, residential fridge w/water and ice, and the outdoor shower and washer/dryer prep. It takes 3 gallons, cold and hot lines together, just to get the pink pop to the outdoor kitchen at the back of the trailer. Another 3 gallons to get thru all the faucets and I to the low point drains. Then I use 2 more gallons to put into the 5 waste tanks. 8 gallons total. The way I see it, $35 /yr in pink pop is cheap insurance compared to a frozen line or valve on a waste tank in a $100k+ trailer.
2017 North Point 375BHFS
2015 Silverado 3500HD Crew Dually Dmax 4x4