A family of 5 can go through the water a lot faster than one realizes.
Biggest uses are running faucets for hand washing/rinsing and showering. Unless you catch all the water used in a large bowl or the sink more water may go down the drain than you realize. A lot of it wasted just waiting for hot water too.
If you are running off your fresh water tank with pump, take into consideration that the average water pump in an RV delivers 3 gallons per minute. Faucet ends may have flow limiters in them (aerators) that limit flow to around 2 gallons per minute.
A minute here and a minute there adds up. Biggest hassle I had when raising kids (and with an ex-wife) was getting them to to wet their hands, turn off water, soap and lather, then rinse as quick as possible. Usually they would turn the water on just like at home and let it run throughout the washing process.
Dish washing can consume a lot of water too if lots of water is run for rinsing. To conserve in my TT I wash from a large bowl with soapy water. Place all washed dishes and utensils in the other sink. They all get rinsed at once and put in the dish drainer.
About half a gallon in the wash side and maybe a gallon to rinse all at once. A far cry from estimates that people use from 8-24 gallons of water when hand washing dishes at home.
In closing remember that tank gauges read in 'thirds". Your gauge may have dropped to 2/3 rapidly but that just means that the water had dropped to just below the full indicator. The tank could well be over 90% full although the gauge indicates 66%.
Same when the gauge shows 1/3. Just means that the tank is just under 66%..
Gauges have a 33% spread from one reading to the next so don't be alarmed. Even Empty can leave you with 1/4 tank or 10 gallons.
Welcome to the wonderful world of RV tank gauges