Copyboy,
I am suprised to hear that you are only getting about 23 in the FEH. I was able to get 25 driving from Pittsburgh, PA to Stockton, NJ, fully loaded, with a roof top carrier packed to the gils.
Does the car go into autostop? If it does, have you experimented (maybe in a parking lot), with driving only on EV? It took a few times for us to figure out how to run the car on the battery only up to about 35
mph.
Why the techs at the shop may not have found a problem may be an experience issue; they may not have the experience of driving the vehicle on a regular basis, and may not notice a problem that a hypermiler would think was obvious. (Makes you think how jet aircraft mechanics fix airliners when they are not pilots.)
If you are not keeping a mileage log, I'd suggest starting one. If you are, maybe you can talk out a typical trip with someone here. There are some ultra hypermilers (nudges xcel), who may be able to point out an area to investigate.
Don't give up on it yet.