Buying a Tanker Semi Truck Trailer in a Small Fire Station is impossible because you require a "Crew cab semi" which, for some reason, cannot be purchased in a small fire station unless you have a "Mass Casualty Trailer Extension." The Mass Casualty Trailer Extension cannot be built on a small fire station as that is a ambulance related extension.
These issues do not occur in a "regular / non-small" fire station because the Mass Casualty Trailer Extension requirement to buy a Crew Cab semi does not exist in those stations.
Seems like the developer tried to prevent people from using crew cabs in small stations but did it in a very odd and confusing way which makes it seem unintentional and like a bug.