I just went to the Ameri-Kart website and the first sentence on their home page is: "Ameri-Kart is the Recreational Vehicle market's leading manufacturer of RV parts and tanks."
http://www.ameri-kart.com/RV-Products.aspx
They provide tanks, fender skirts, sinks and many other plastic components for the entire RV industry. Their tanks are found in many more than just Keystone RV's. That said, I haven't heard of an increase in water tank or holding tank failures over the normal "one or two a year" that are posted on the forum.
I've said a number of times in previous posts that dumping chlorine bleach "straight from the bottle" into any plastic tank is not good for the "essential plastics" of the tank. In the past I've seen some extremely "nasty" fresh water tanks that have grown black mold from not being properly cared for. Some of the extreme measures to clean those tanks have included acids, bleach, detergents and even mechanical scrubbing, none of which would be considered "good for the tank's longevity"....
That said, you might want to talk to the previous owner (if you know who it is) and see if there is any history of chemical cleaning of the tank. Hopefully the previous owner will be honest in his response.
Again, I don't know of any "broad problem" with roto-cast plastic tanks from any of the manufacturers. There are always "isolated problems" but nothing of any magnitude.