The WH capacity is usually included in the advertised water capacity. If you look on the sticker for camper's GVWR and CCC, the weight of all water is calculated and subtracted to determine the CCC.
Also 27 USG is 22.5 imperial gallons (1.2 USG = 1 imp. gal).
On another note, you'll never be able to draw the full capacity from the FW tank as the supply for the pump is on the side of the tank. There's usually a sump and the FW tank drain is off of that. Some has switched the pump supply to the drain sump to be able to use the tank's full capacity.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63