Drop in gas mileage

I recently got a new used car that was my dad's business vehicle, and I am experience fuel economy issues.

Right before I bought it, the check engine light came on and my dad took it to the shop. They said it needed new spark plugs and wires. A few weeks later, I took over the car, and it went from its normal 22 - 23 to about 12 mpg. We changed the plugs and wires and it seemed to fix the problem as I got 26 mpg on the way back home. However, the problem is still there. It seems when I drive the car from in one trip, I get really good fuel economy. When I drive the car over the period of a week or so, I get poor fuel economy.

I think that the problem may be the fuel pump or a leak in the tank or lines somewhere. But, before I go all gung ho about that, I wanted to know if I should try something else. I was looking at those fuel injection cleaners sold at department stores and I thought I might try one of them.

Any suggestions?

I have been having poor gas mileage for a few mos. Now. Also, when I accelerate going up a long hill or incline the engine "misses or lurches", also does this sometimes when I am at a stop light. I changed the air filter, had a tune-up including new spark plugs, spark plug wires, fuel injection cleaning. No improvement. Then there was an episode where the engine light came on and it was almost like it lost power, engine missing big time. Took it into the dealership and they ran a diagnostics. They replaced the cam/crank sensor and also did a software update on the main computer of the van. No change again. Then the engine light and power loss thing happened again. The dealership said it gave off the same codes, but since they had already taken care of those two issues, they test drove it. Said they did not notice any missing. They now want to have it for the day to put in on a "flight plan" (? ). I have said to everyone who looks at the van that it seems to be something with the gas and/or emissions but no one seem to go in that direction. I have had a few ex-mechanics mention the EGR sensor or valve and also the gas pressure regulator (?). What direction should I be heading with this issue. I am getting tired of running it into the shop and have spent around $1000 now with no improvement. Thank you!