Re: Islander 28 waste tank
Our 1979 I28 still had the factory installed "Firestone" bag. Lord was it a stinky pig! I purchased a NAUTA (sp?) bag, additional fittings and installed it several years ago. I got a great deal on 12 gallons but if paying full price would go up one size.
Bags do want attention while pumping out if you want a good job of it. Just pumping will empty it most of the time but not completely. I always go for the flush rinse and second pumping... During pump out, I do lift the bag to puddle its contents under the drain fitting.
The bag is OK but I do wish I had gone with the absolute premium hoses, the standard quality product will become saturated and begin to pass odors.
So far, a "No Odor" (branded?) powder additive seems to work best keeping odors in check. That product came as a box of packets. Sorry the name escapes me now.
I also ran an extra long vent hose and included a loop to the top and most inboard parts of the head storage compartment. Since doing that, I have never filled the holding with water on a hard-driven port tack.
There is one other thing I would change if in a warmer climate. I'd add a treatment system to the flush water intake or install a tank for shore supplied flush water. I believe the lake water is a major source of the aromatic activity in a holding bag/tank...