Moderators, consider moving this thread to the
electrical category.
Andy, Locate the fuse for the fans.* Pull the fuse out. Switch on the fan (any one). Check for voltage at the two terminals where the fuse would normally connect.
With the fan switch in the on position, the
electrical circuits are complete except for the one break you made by pulling the fuse. One side of that fuse terminal is connected to the 12 volt +. The other side of that fuse is connected through the switch and fan motor windings to the battery's -. Your meter will be measuring the difference.
If you don't have a difference of 12 volts with this check then there's another break (other than the pulled fuse) in the completion of the electric circuit.
If you do have 12 volts, then the circuit integrity is complete and you can start looking elsewhere (fan physically jammed?).
Since all fans have stopped functioning at once, I'm more inclined to suspect a shared cause rather than mud daubers or wasps building nests in each all at the same time.
Jon MB
* The presumption is that ONLY the fans are on this fuse. For the circuit integrity check to work EVERYTHING powered by this fuse will need to be physically switched to off.