[A/S] blackwater tank smaller than listed

[Fritzs@xxxxxxx]

Hi all,
We took our shakedown cruise in our new-used 97 A/S Safari this 
weekend, and noticed that the black tank gage was showing that we had 
a half full tank after an hour. I'm not that full of it, so after 
dumping the tank, I poured 5-five gallon buckets of water in, and 
filled it up. Where did the missing 15 gallons go? Apparently, 
according to the dealer, I've got paper mache in there.
How do I get it out?
Fritz & Carol