Low tire pressure(s)? Check them, they should be around 35psi, 32psi minimum. I run my TCH tires at 40psi. Driving from San Diego to Phoenix Arizona and back I got 40mpg with the AC going - it was about 100F that weekend.

Unlikely but possible is a dirty air filter. I assume your speed was about the same as when you got 36mpg.

The OBDII system will cause the check engine light to come on if there is anything odd going on. In the absence of a check engine light, I doubt the dealer will find anything. Most likely they will "clean the injectors" and say nothing is wrong...

Good luck. Let us know what you find out.