Spec sheet for the 20RD shows 42 gallons for the fresh water tank, but that's still significantly more than 27 gallons.
Not familiar with your line of trailers, but I'd check for a leak / crack in the tank, or a problem with one of your upper lines (vent, fill, or return line if applicable).
Another very far fetched possibility is they installed a smaller tank from one of their 16 foot trailers.
I havenít been everywhere, but itís on my list.
2021 Geo Pro G16BH
2012 Nissan Armada 4x4 Platinum