have you tried to call the factory. they might be able to look up your build sheet with the VIN number. I don't know how long they keep the paper work on units, when I asked what size tanks I had they were able to provide me with an answer. Also can you see your tanks? if you can see them try to get a measurement and calc it that way. you could also add water 1 to 5 gallons at a time and see what you get.
2007 Chevy 2500HD CC
2010 V-Lite 30WRLTS
Nights Camped 2011 -64
Nights Camped 2012 -50
"I Live in My Own World, But It's OK. They Know Me"
2011 FORD F350 CREW CAB DUALLY DIESEL 4x2
2010 Salem LA 292FKDS
One of the Original 14 F.R.O.G.s
1st, 2nd, and 3rd INTERNATIONAL F.R.O.G. RALLY Branson. MO.
1st, 2nd, and 4th INTERNATIONAL RALLY Goshen, IN.
I had a 2005 Wilderness 27FBSS and still have the 2005 Brochure, It list a 26BHSS which is probably the same as your 27BHSS. All models at the time had 50 Gal fresh,30 Gal grey and 30 Gal black.
Hope this helps.