Is it legal to film a police officer inside of a vehicle and officers/ detainees only section of a police station if one were arrested?
TLDR: are police vehicles and the inside of a police station public places?
It is not decided whether such, in areas, filming can be prohibited. In a "public forum", First Amendment rights are maximally protected, and this includes filing (Glik v. Cunniffe, 655 F.3d 78 in particular §A(1) for a summary of the law on this question). In Perry Educ. Ass'n v. Perry Educators' Ass'n, 460 U.S. 37 the court refined public forum doctrine to distinguish quintessential, limited, and nonpublic fora. In a nonpublic forum, the government may "reserve the forum for its intended purposes": but, such a reservation must be viewpoint-neutral. Grayned v. City of Rockford, 408 U.S. 104 states that "The crucial question is whether the manner of expression is basically incompatible with the normal activity of a particular place at a particular time". In a nonpublic forum, demonstrations can be forbidden, especially when demonstrations are incompatible with the purpose of the forum (example: military bases).
The particular question you raise has not ended up in court. One may be tempted to reason that First Amendment rights can be arbitrarily curtailed in a non-public forum, but such a restriction would have to have appropriate justification – the restriction would be subject to strict scrutiny. Simply saying "We don't want people filming inside the holding area" is not a valid justification. But, the police are not required to announce their legal argument in advance. If you get busted and film, and they make you stop, you can sue for violation of your First Amendment rights. Your attorney would then need to make a good argument that this restriction is to be subject to strict scrutiny, and that it fails.