Drain your tank using faucet, (kitchen or bath)
Fill up one gallon jug and empty into tank. Count how many times you can empty jug, repeat till water tank is full.
That is how many useable gallons you have.
One of three things...
1. If you can't repeat 30 times...you might as well have a 1000 gallon tank, if you can't use it...doesn't matter. Water exit on tank to plumbing pipes are up to high and the water pump is not able to suck out all water.
2. Your 30 gallon tank is what's advertised but really you have a smaller tank. And somewhere in manufacture advertising there is a little asterisk. In another spot look for words that reads "subject to change"
3. 30 gallons is only six big water bottles, learn to conserve, which takes awhile and source of "marital bless"
