Yesterday someone here wrote that the text about Yosemite was a red herring because they were nowhere near that park or California at that time. I totally agree. But I'm not sure it was intentional on BL's part. I think he made a big mistake.
Assuming her mother's intuition is right and Gabby didn't write that message, the fact is that the van was close to Yellowstone, not Yosemite. Something had happened to Gabby and she was not there anymore, and BL was in a panic. He knew Gabby spoke to her mom all the time and he wanted to avoid questions after "whatever" happened to Gabby happened.
So in my opinion, he sent the message "No signal in Yosemite" to throw off her mother, but he screwed up and wrote the wrong park name. A blip of a frantic mind or a guilty conscience? I think it's a clue to his state of mind.
Another possibility. At the very end of the LE video, when Brian is being driven to a hotel for the night, the cop talks to Brian about places he thinks the 2 of them really must visit. Yosemite was one of those places. Maybe that's why it was on his mind...