Does anybody know how/where to store The Liberator in GTA V story mode? I tried in Franklin's garage as well as Trevor's and neither worked.
No need to store it in the garage, you should be able to select special vehicles at some point and it should be listed.
Found in each protagonist's garage in single player in the Special Vehicles section. It, along with the Sovereign, were no longer available as of 8/5/2014 due to a glitch. Both vehicles returned to the garages after the San Andreas Flight School Update (XB360/PS3).
Special Vehicles are features in the Grand Theft Auto series, this article contains a list of all the special vehicles featured in Grand Theft Auto V.