Mine was supposed to have 45 gallons each fresh, gray and black. Doing an inspection one day I noticed my gray was much bigger than the black so I measured them. The black is 45 gallons as stated, my gray is actually 60 gallons. It's wrong on the label and the books, of course it's wrong in my favor so I'm cool.
Best thing to do is measure them. W X L X H=capacity in cubic inches, inches/231=gallons.
2001 HO Cummins powered Dodge 2500
2014 Sierra 346RETS
Nights camped in 2014-28