I need help, possible gas leak.

I have had my Kia Sedona for 4 year and until recently have not had any issues with gas mileage. Typically a full tank would last me two weeks. In the last couple months I noticed that when I fill it up I burn through the first half of tank in 2 days. Then the second half in roughly a week. I was told that it was probably because I was remote starting it or I needed new plugs. Had plugs replaced and stop remote starting it, didn't help. I didn't smell gas, nor did I see any puddles under my van.
I decided that I was just going to fill my tank up 1/2 way from then on. It seemed to be helping, but then over the weekend I put in half a tank and within a day I was down to under a 1/4. I only went to work and back which is about 10-15 miles, all highway.
Called my mechanic and he said that without the check engine light on there is really nothing they can do. Told me to take it to my dealership, which I refused to. Last time I did that I somehow got suckered into putting a new engine in it, more money to fix the thing then it was worth. Please help, what could be causing this, and how do I stop it short of getting a new vehicle. I can't afford to waste the gas or get a new car.

I have had my Kia Sedona for 4 year and until recently have not had any issues with gas mileage. Typically a full tank would last me two weeks. In the last couple months I noticed that when I fill it up I burn through the first half of tank in 2 days. Then the second half in roughly a week. I was told that it was probably because I was remote starting it or I needed new plugs. Had plugs replaced and stop remote starting it, didn't help. I didn't smell gas, nor did I see any puddles under my van.
I decided that I was just going to fill my tank up 1/2 way from then on. It seemed to be helping, but then over the weekend I put in half a tank and within a day I was down to under a 1/4. I only went to work and back which is about 10-15 miles, all highway.
Called my mechanic and he said that without the check engine light on there is really nothing they can do. Told me to take it to my dealership, which I refused to. Last time I did that I somehow got suckered into putting a new engine in it, more money to fix the thing then it was worth. Please help, what could be causing this, and how do I stop it short of getting a new vehicle. I can't afford to waste the gas or get a new car.
Go to an emission testing shop and get the exhaust gas analyzed. This will give you areas to check. The amount of extra fuel used will surely show up in the test results.