Mods feel free to merge this with the larger US Navy in Twilight 2000 thread we've had coming and going.
Mark Felton is a youtuber with an eye to military history. He recently produced two videos talking about ships built during WWII, still in use today.
Pt. 1:
https://www.youtube.com/watch?v=9nAGbsjM5CE
Pt. 2:
https://www.youtube.com/watch?v=UjUha9XBZAE
More than a few of these vessels are from the US (mostly coast guard tugs and buoy tenders), and were not transferred until the 1990s or 2000s, which means in the event of a diminished US fleet, post-TDM, they could potentially be pressed back in to service rather than being sold.
Perhaps most interesting is this vessel, an un-powered barge:
http://www.navsource.org/archives/14/5926.htm
Essentially a floating hotel with machine shop, hospital, rest berths for crew and a helicopter platform.
I would wager most if not all of these would be held for coastal defense, only capable of mounting ad-hoc weapon systems (perhaps a stock of Hellfire missiles, turning them into light missile boats ala the OSA-II class, or, given enough time, perhaps even Harpoon rails, but more likely the former).
Just more grist for the idea mills, gents.