After a quick search i find the brochure states 40 gal. but if you do a search on here you find back in 2016 someone asked the same question. Seems that the contents of the gray tank fit in a 27 gal. blue tank.. Maybe a misprint by the manufacture?
2009 Seville VF32KS
2019 GMC Sierra 3500HD Crew Cab Denali.
6.6 Diesel standard box.