My in-laws just picked up a Windjammer 3008W. It has a front kitchen with a 38 gal gray and another 38 gal gray for the bath. It is listed in the specs for their TT as having 78 gal gray. I bet your tank setup is similar...
Russ & Kim
2014 Alpine 3600RS
2007 Dodge Ram 3500 5.9 4X4
2008 Harley Davidson Road King 105th Anniversary
(2011-5 bad year)(2012-40)(2013-45)(2014-23)