I somehow remembered, at least in Washington State, it is required to have documentations for your "service dog" But it is illegal to ASK for paper, which means you can totally refuse to show any certificates, the laws worded it in a very confusing way I think. In the case of bringing dogs to national parks (most of national parks in any States do not allow dogs) in WA, the rangers are not allowed to ask for proper paper for service dogs. I might be wrong but that's what I recall.