OP, I would not dismiss the possibility of leak in your supply line based on the rate of water loss. I also saw a doubling of my water bill, but it was the start of the summer and initially caulked it off to my wife's enthusiasm for grass/landscape irrigation. And we never noticed any change in the water supply of the pressure. But then I noticed a small but persistent trickle of water from my gutter drains going into the street and realized I must have a leak somewhere. A little sleuthing made it quite obvious that the leak was in my supply line pipe that runs under the concrete driveway.
I contracted the repair to a company that could replace the old plastic pipe without tearing up the concrete. They dug small trenches to expose the old pipe on either side of the driveway, snaked a cable through the old pipe, and then pulled (with a pickup truck) a knife-like pipe splitter (pushing the halves of the old pipe to either side) followed by a length of new (supposedly better) plastic pipe. They made the water connections to the new plastic pipe, refilled the trenches, and we were back in business. As I recall, the cost was around $1500.
Good luck!
P.S. - This thread would be better placed in Home & Garden