Welcome to the forums !
Here are some tips that may help :
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
65+ Vehicle modifications for better fuel economy - EcoModder.com
Since you live in the mountains, you have the advantage that despite having to climb mountains, you have the other side in which you can coast down for miles. If you could find a route that has more downgrades than inclines, you might get better mileage even if the route is actually farther to the destination. Since your car is an automatic, I'm going to let the experts take over on advising you on the best way to do this.
As far as aeromods, your car is relatively sleek from the start. I can't find an exact figure for the car, but most cars like yours fall around .31 -.33 region as far as the drag number goes.
You could probably benefit the most from a full bellypan under the car ( with metal used around the hot areas )
The Lexas LS 430 had a .25 drag coefficient which is equal to the most streamlined cars mass produced on the road in the last 50 years. This is mainly done with careful tweaking of the airflow around the car. A large part being the smooth underside of the car.
Note how that despite being so sleek, the car looks average - even boxy.
There are no boat-tails used on the car. No wheel skirts or anything either.
What this means to you is that with careful testing ( A-B-A coast down testing, tuft testing ) your car could match this figure, yet look absolutely stock to anyone that didn't know.
The only person to know would be someone that looked under the car and could see the bellypan . You can also have a fully 'stealth' grille block that goes unnoticed.
Good luck !