Owner installed fittings in bladder tanks are too often inclined to leak--the last thing you want to happen in a holding tank. Plus, the average lifespan of most bladder tanks is only about 15 years...whereas hard plastic tanks are pretty much
indestructible forever as long as they aren't abused (fittings overtightened etc). So I strongly suggest that you go with one instead of a new bladder.
Ronco Plastics
Ronco Plastics marine Tanks (no relation to the VegoMatic Ronco) is your best source for a tank. They make TOP quality thick-walled water and waste tanks for a very reasonable price (prob'ly less than the same size top quality bladder) and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank. We might want to chat one-on-on about the best location for it (which may or may not be where the bladder is) and also fittings locations and vent sizes, so feel free to send me a PM if you'd like my help.
Oh...you prob'ly should replace all the hoses too...permeated sanitation hoses are the most common source of odor inside a boat.
--Peggie