Fred,
From your description, I can not tell if you really have a problem with your air tank system or not.
The service schedule that is given for the air dryer is based on a commercial application that the vast majority of them are used in, and in my opinion it is far to aggressive. With the irregular and limited use that most coaches receive, I prefer to use the efficiency test as an indicator for change intervals. Drain your air tanks at the bleeders in your LR service bay every week or two when the coach is being driven, and if you get any water from the tank drains, you need to service the air dryer. If you never get any amount of water at the tank drains, the dryer is doing it's job as designed, so save the time and money to fix something that is needed.
Gerald