Heh heh, I have an odd arrangement. My P30 has no real holding tank. If it did, it's gone now. The PO plumbed the toilet discharge to a 5 gallon, plastic fuel can.
When it's full, I just disconnect it and put it in the back of my truck and take it home, where I pour it into my home's sewage access point in my back yard. (I wear chemical protection gloves to ward off any splash that might occur) My house is 20 seconds from the boat, so this isn't a real hassle.
The only downside, is that 5 gallons isn't much capacity.
S/V Old Shoes
1973 Pearson 30 #255