I think you are the right path to try to figure out the grounds. Typically those are not far from the appliance or device that they serve. I'm assuming that there is wire loom and that if you opened it up from the air horn compressor back, you might get a hint as to where the ground connection is. I'd start with that one. Depending on how that goes, I'd tackle the air ride compressor the same way. Make sure that you have a good, solid ground in both cases and then use that to confirm that you are getting 12 volts to each device.
For the fans, I'd try to take the wiring connector near the motor apart - carefully, it may have a hot lead. There may simply be a crimp connector or a wire nut that is loose. The reason that I would start there is to verify that when you turn the fans on, the one that isn't turning is getting the 12 volts - or not. If yes, the motor itself is bad. If not, you have a connector problem and I would start at the switch that turns them on.
On ours, there is a good sized bolt near the house battery compartment where all of the major grounds are tied to the chassis. If you find one of those, I'd suggest removing the nut (or nuts) that hold the wire on and remove and clean every connector. It only takes a little moisture to have gotten in about half way down the stack to give all of the wires above it a poor connection.
2000 Georgie Boy Landau 36' DP
2005 Saturn Vue toad