My 2013 Montana 3402RL Does Not Have a Gravity Fill.

If you fill your 25 foot hose, that is 300 inches, or 300 X 2.5 cm.. about 750 cm.

Your 1/2 inch hose is roughly 1 cm across, or 1/2 cm radius.

The formula for the volume of a cylinder is Pi X R(squared) times length.

So, your hose will hold 3.14 times 1/2X1/2 times L

Roughly 3/4 times 750... about 600 cc. A cup is 250 cc, so you might want to measure how much you put into the hose.