You think that a water heater can make a $70-80 difference? Everyone I
talked to said no way. I'm thinking about replacing the water heater
anyway and see what difference it makes.

As for the gas leak, what if it was underground? I'm on a slab (both
old and new houses) and I've been wondering if a leak under the slab
wouldn't be as detectable.

Another way to check that would be to have a pressure test done.

And to Tom's reply, If it was an estimated bill it would eventually
catch up with me and I'd eventually have a low bill. In living here
2.5 years, I've never had a "low" bill.