As you struggle to find the porcelain throne has your water bill dropped any since your first post? Have you looked for leaks?
Your house inspection probably found no leaks but just to be sure check your water meter. Home inspectors will also do this. Have all faucets and appliances off. Looks at the meter is you see water flow. A mechanical meter will have a little star wheel in one corner and it turns as water is drawn. A digital meter will show an arrow flashing but you will need to look at it for about 30 seconds or more to see it flash. If there is any movement you have leak somewhere. The old flappers in the toilets is the first place to look but you would here the toilet refill it self as the water level dropped.
Another common source for a leak is from the meter to the house. If Birmingham has similar weather as Florida (temperature wise) you might have PVC pipe from the meter to the house. In Florida the meter is near the road or at the sidewalk just below the ground. One cause of slow leaks is the PVC pipe with be lifted by tree roots and then cracked. Another is the threaded nipple right at the meter. I've had this happen to me, my neighbor and my son's house. I also fix this a friends daughters house. She got a new place and always complained that her water bill was awful high for a town home with one person.
Living in S. Florida for 25 years and having many new younger neighbors moving in, they all seem to have the same complaint, MY WATER BILL is HUGE. Our homes were build with 5 gal flush toilets with at least two or three in every home. Many, including myself, have changed them to water efficient units. But the number one reason for high bills is watering the lawn. It always seem that new owners want green lawns and the sprinklers are running constantly until they get the first bill.
My biggest water saving device was getting a HE washer. The old top loaders used 20-30 gals per wash, my Samsung HE uses around 11 gals per wash. It's only my wife and I and I wash my works clothes each work day (5 days) plus regular wash of other clothes, towels, etc. Our gallon usage is 2000 or 3000 gals per month. I have a well for irrigation. If you have kids, that is a given that you'll have high water bills.
I once read an ad on line for a plumber that said, ...where a good Flush is better than a Full House".