2.5G/day doesn't seem too far off for that size water-heater and daily cooking. Where's your water heater? - In the garage, I assume, where it can get chilly - and that's a good-size amount of water to keep hot (it's about twice what mine holds - but we don't need big ones in FL, as the ground water is never less than 70 degrees or so). Not to mention cooking eats up quite a bit of gas. I know when we cook a lot in a given month, I notice a noticeable increase in our bill. Propane provides about 92,000 btu's energy per gallon. A typical 75-gallon water heater consumes about 70,000 - 80,000 btu's per hour of operation. If I had to guess I'd estimate that my water heater probably runs for about 15 minutes at a time, and it maybe kicks on every 2 hours or so. Also, pilot lights use up more than you may think.
So, it's not unreasonable for the water heater to use a bit more than a gallon per day of propane, maybe even 2G/day in colder climates. Add a couple stove burners running a couple hours a day, and there you have it.
Nevertheless, if you even suspect a leak, get it checked.