Expanding on comment above about some Airstream trailers having only one combo tank, I also noticed in Noverber 2016 MotorHome Magazine that the 45-foot luxury Foretravel DP has freshwater capacity of 135 gallons, and
"Black/Gray Tank Capacity 145 gal (one holding tank)"
With newer dumping technologies maybe it makes more sense to consolidate the black and gray into one. It would not surprise me at all to see a trend in this direction because it solves other problems as well.
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