Ben,
you`re right Headhunter is very supportive in that they will sell u the complete unit only for $508/ tank sentry , no repair parts because it`s a sealed unit ! Ouch. All three of mine went south even after trying the recalibration steps ,etc. I`m only replacing the black tank for now with the logic that fresh water will fill as needed and shut off when connected to water supply and i know from usage abt when grey tank needs a dump. Fresh water fill valve was removed, cleaned and nothing, needs replacing also $189.