If memory serves your tank is under the bed of your truck and your compressor is on the frame rail (which affords the compressor some protection that your tank and its fittings, including the sending unit and solenoid, probably lack). This begs the question of how exposed your sending unit is to salt, brine, water, sand, road debris, road tar, and other such things. If the sending unit is on the topside of the tank, that probably helps protect it from heavier things like debris and tar, but lighter stuff like salt, sand, brine, and water can probably still get to it.
In such a case, I would expect to go through multiple units unless the one that was installed was very well protected from the elements. As an example, by ‘very well protected’ I mean: all connections heat-shrunk and any exposed metal connections covered in dielectric grease … and periodically checked and re-coated. Another option for ‘very well protected’ would be enclosure of the sending unit within an environmentally sealed enclosure (much like the enclosures that most vehicle computer junctions have under the hood).
What I’m getting at is that I’m not convinced (yet, because I don’t have enough data) that the sending unit is the problem in your case – your issues could well be caused by exposure to the elements. Granted, I avoided Air Zenith products because I had read about issues with them, and my Dakota Digital sending unit and gauge combo has been flawless for ~18 months. However, to be fair, my Dakota Digital sending unit sits high in the engine compartment – well protected from road debris, salt, brine, and other such things. All of its connections are soldered & heat shrunk, and all exposed connection metal is coated in dielectric grease – so the worst it has to contend with is engine compartment heat – which my ad hoc temp readings have shown is within spec for the unit.
As with anything electric on a vehicle, placement and exposure to the elements/hazards plays a role in lifespan…
My apologies for deviating from the blinking 220 PSI topic – but I felt this was worth pointing out.