I use a 35 gallon bladder tank for my black water. It is installed on the centerline under the cabin sole, next to the head. It has given me no trouble, and has the interesting feature that it needs no separate vent. When the marina sucks the stuff out, the tank goes flat. As the head is used, it gets bigger and bigger until finally it won't work. It is vented through the cap on the pumpout - I drilled a small hole in it and put in a venting filter made of brass.
I plan to put in another bladder in-line under the sole, to hold 30-40 gallons of water. The off-center water tank drives me nuts, and I can use that space for other things.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida