Lots of times the holding tank can be built in at the factory. Meaning it won't fit out of the space that its sitting in. I had to remove an old holding tank on a Crealock
37 last fall. Nasty Job. Best thing to do would be to flush it with fresh water
and bleach about 6 times. Slide a couple contractor bags around it and cut it into pieces with a sawz all. Put on a tyvec suit and rubber gloves.
Also it is worth ordering a gallon of pure arye. I have no affiliation but found it to work
well. follow the directions and clean all surfaces of the locker and head with it.