I don't really have _THE_ answer, but have almost the same problem with fish processors, and we find that first using a dab of dielectric grease to the plugs and pin side of the connector, then sealing each and every connector outside with RTV Silicone that is rated to not cause problems with O2 sensors (we use the clear, we feel like the other colors may cause added problems) is about as good as we can do.
The trucks never last 2 years anyway, so we just tell the customer up front that 2 years and his trucks will be ready to add to the pile of trucks going to the smelter. I bet the fish plant leaves a smell the salt plant doesn't have to deal with. and the fish plant trucks are always battery trucks, not IC.