Agreed. The residential plumbing codes absolutely prohibit ABS plumbing for carrying any water under pressure, because it has a poor reputation for longevity. For storage of potable water it is perfectly fine. For drain and vent systems, ditto.
There is no valid reason to not use the existing tank if it hasn't been contaminated. Even if it has, it would be possible to sterilize it.
I plan to plumb mine into the fresh tank, I just haven't quite figured out the details. The huge advantage in working with an ABS tank is that you can drill a hole and simply glue in a nipple for a vent or overflow line. For the polyethylene fresh tank you should spin weld it, which is problematic for an owner.... tho any RV shop should be able to do it if there is decent access.
Tons of great info about repairs ... http://rvsandohvs.blogspot.com/2012/09/holding-tank-repairs.html
BTW we are figuring out options and will get a deposit off to the factory really soon.
John Davies
Spokane WA