While a large part of my research leads me to believe that this is literally a huge mistake on the part of Frost and Lynch, the location of Deer Meadow still bugs me, so I'll briefly go over why and maybe someone knows better then me:

Teresa Banks was found murdered "in a town in the Southwest corner of this state" wrapped in plastic on the shores of the Wind River, which is consistent with the geography. Agent Desmond flies to Portland, Oregon, which makes sense considering that Wind River is just over the state line in Washington, and heads to Deer Meadow to investigate Fat Trout Trailer Park and Hap's Diner, where she lived and worked, respectively. This, however, doesn't make sense with the narrative. I could easily see Leland traversing the state to indulge in BOB's desires, but I doubt Laura Palmer or Ronette Polaski would have managed to simply casually travel to the area, and I don't see how Teresa Banks would have any connection to Twin Peaks, yet she is working for/with the Renault brothers.

Here's where things get tricky: The Secret History of Twin Peaks states that Deer Meadow is in the neighboring county "just west" of Twin Peaks. Also, we know that the trailer park in The Return is called "New" Fat Trout Trailer Park, and is indeed a different location then the first. Now, Wind River DOES converge with the Columbia River, which makes it's way throughout Washington State and into Canada, so it might be likely that people in the region consider it to be synonymous. Perhaps the generations in the area before had used the Wind River/Columbia River without differentiating between the two?

Even more tricky: The Secret History of Twin Peaks has an article about Andrew Packard's experience in the woods as a Boy Scout, and he specifically talks about Fat Trout Stream being in the general area. Carl, however, was not the original owner of Fat Trout, he became a manager and later came into ownership.

The only "theory" I have for this is simply that of metaphor. Twin Peaks is at the literal opposite side of the state, and Deer Meadow is supposed to be the "opposite" of Twin Peaks. Other then that, I believe this to be a major mistake on the part of Frost and Lynch, but if anyone has any thoughts they'd be much appreciated.