While surveying/measuring for replacement of hoses, valves, etc. on our Columbia 9.6 project boat we openened the inspection port on the holding tank and it was 3/4 full. We are having it pumped out this week, but: The tank is a plastic material - does it absorb this stuff over time (10 - 12 years) and should the tank be replaced? Sealed with something after cleaning? Taken out and inspected? Our intention has been to replace the entire system (including the head) from the holding tank back. Thanks!! JT