I suspect your waste bladder was a replacement.
While I don't know what how waste management was done in the I-28 originally, I do know that Islander used quite a few plastic tanks both for water and waste. Many times they used tanks from Ronco Plastics
, which is still in business. I have found Ronco to be quite helpful when I had questions concerning my water tank.
If you want pumping to be a one man job, I'd recommend doing some careful management under your settee and perusing the Ronco Catalog
for a tank that will fit. I was astounded at how many tank designs they have. If none of those work for you they do custom tanks as well.
Take a look at B145 in the PDF catalog (it's easy to search for B145 in Adobe Acrobat reader). It fits nicely under my starboard settee and might fit yours.
Overall, I'd wager you could replace your bladder with a tank and fittings installed for less than half a boat buck.