Since Season 8, a Prototype Exploration Unit is being unlock from the start, normally player has two possibilities to unlock it, either on the course of doing mission Forgotten Prototype in Kola Peninsula or by finding it near a station in Don region.
No changes were made to the unlocking properties of the trailer and possibily any unlockable trailer now is by default unlocked (but the game of course has only one such trailer and that's the mobile watchtower). After the first release of Season 8, there was a bug that unlocked all the upgrades, it wouldn't be surprising the same happens to trailers.