I just checked the listing on Amazon....those tanks can't be top quality or even decent quality for that price....at best on a par with Plastimo. "Nautos" is apparently an attempt to mislead people into confusing it with Nauta, which is top quality...check it against Defenders price for a Nauta 35 gal tank. Vetus bladders are mid-quality at a somewhat lower price.
The average lifespan of most bladders is only about 15 years before they start to leak at a seam or fitting...or chafe through unless they're carefully protected from chafing when installed. Like most things, the better the quality, the longer it should last.
I know you don't want to cut into anything to install a new tank, but when you compare the price of a good bladder with the price of a top quality plastic tank, you may decide it's worth the effort. So check out Ronco Plastics... They make TOP quality thick-walled water and waste tanks for a very reasonable price 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, so you may find a shape size that can be swapped out with a minimum of fiberglass surgery. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with.
Ronco Plastics marine Tanks is their current catalog...most people find their previous catalog easier to search so they've left it up
Ronco Plastics old marine catalog
--Peggie