There aren't many dining options between the Columbia River and Spokane, whether you take the slow scenic route (US 2) or the fast and fugly route (I-90).
You're looking at eating at sketchy no-name restaurants or chains. However, occasionally there are diamonds in the rough, and we found one at El Ranchito.
We pulled in here on a hot August day on our way to Montana. We could have lasted until Spokane, but decided to get an early lunch.
While El Ranchito's menu is mainly "standard Mexican food" (combinations, enchiladas, and a few Mexican specialties), the execution is top notch and the service is swift and friendly. I got the chicken enchiladas and my dining companion got the vegetarian (spinach) enchiladas. Both were very fresh and good. Moreover, the prices were 15-20% less than we'd pay for equivalent Mexican food in Central WA (Wenatchee, Yakima) or Seattle for that matter.
If you're passing through Davenport and you like Mexican food, GO HERE. That is all.