If you can’t find a leak, you can try a tune up. New O2 sensor, plugs, cap, wires, air filter.
A leak bad enough to halve your mileage should be pretty obvious. The filler tube on my wife’s old car rusted out and had just a slow drip when it was completely full and you could smell the gas standing anywhere in my front yard.
So if it only does it while it’s running what I’m trying to say is you probably would have burst into flames by now.