We've only owned the camper for a couple of months, but it's 14 years old. Don't know for sure if the gauge is accurate, but it seems to be. I did a "test" last week, tried putting two bucketfuls of hot water down the sink and then measuring what came out. All the hot water didn't come out, and the gauge said it was still half full, so I left the gray water valve partially open while driving. The gauge now says it's empty, so I assume it emptied completely during the drive. How would I know if there's any blockage in the system?