Corrected entry: Niles says that after failing to confront Maris, he was turned away from the Oregon border check for having fruit in the car. Oregon is an interior state, and therefore without any form of border check. There might be a border check at the border with California, but Niles is approaching from Washington.
Corrected entry: Why can Niles hear what Frasier has said when he is inside the sensory deprivation tank? The whole point of these are that you cannot hear anything happening outside, so Frasier's ranting shouldn't have been heard inside. Similarly, why did the writers even have this scene with Frasier trying to talk to "Maris" when she was supposed to be inside it? Clearly they didn't quite know how soundproof they are.
Correction: The accusation that Niles could not hear Frasier because he was in a Sensory Deprivation Tank is completely false. Having owned one myself for many years it is true that you can not hear light sounds but if someone is talking loudly enough you can hear them and understand what they are saying to you. This is why earplugs are recommended for people wanting to experience Sensory Deprivation to full effect, as well as keep salt water out of your ears if you have ear problems.
Correction: Niles is referencing the very common USDA produce checkpoint that exists at points along the Washington/Oregon and Oregon/California borders.
I have never once encountered a produce check when entering Oregon from Washington. And Niles would have been on I-5, definitely no such check stations anywhere.