yes, I know the link says 20 gallons, but Inca has their website divided drawings for holding tanks divided into 10 gallon increments. There are multiple cases of tanks in the 30 gallon section being 33, 35, and 37 gallons for instance.
That said, the specific drawing for the H4 does say 20 gallons...
My manual says 25 gallons from the factory. The new tank from Inca was dimensionally identical to the original factory tank I took out. It had two exceptions:
1. the "pilot" marks for the 2" vent line and the 3" inlet connection were in slightly different positions. I ordered the tank without holes or fittings in case this was the case, so it wasnt a problem. The 3" slip outlet was perfect.
2. The Inca tank was black; the original was white.
3. The Inca tank was half the cost (including shipping across country) than buying it from any Airstream dealer who had the tank in stock.
A quick calc of the "rectangular" dimensions provided in the drawing yields an internal volume of 25.8 gallons. From this dimension we need to remove the effect of the slope inside the tank, which is roughly 3.5 gallons, and the volume of the "notch out" by the slip fitting which is about 2 gal. I've rounded and not done super precise geometry, but I think its close enough for our purposes.
So in the end, somewhere around 20 gallons is probably about right. Looks like the manual is stretching the truth a bit!