Tune up

A tune up would be a great place to start. You would look at new spark plugs, air and fuel filters, an engine service comp test, and a scan and report on engine management faults if any are logged, also a live data read to check engine electrics, O2 sensor operation, check fuel pressure is correct. All these can affect the engine tune and fuel economy. Have a chat to your local mechanic and discuss your problems, if he starts mentioning what we have talked about here, he knows what to look for, and just ask for a general quote so you have an idea as to what it will cost. I cannot do that as I do not know what you local labor rate is and what your parts pricing is like, but in general you would expect no more than say one and half to two hours labor max as long as there was no problems along the way.