The installation of my 18 gal blackwater tank was performed while the factory, under their warranty, was replacing the four lower quarterpanels due to corrosion and had some of the working area exposed. I don't have the receipt but recall it was more than $500, which I was told was discounted due to the experimental nature of the work at that time and the hassle factor.
At the time, I did not request installation of the black-tank flush connection. I understand from several other posts that this is a nice feature, but early installations (I don't know if these were retrofit or new installations of the 18gal tank with flush) apparently had occasional problems with leaking in the freshwater flush line.
If you decide to look into that feature, search this forum, including such threads as:
http://www.airforums.com/forum...ush-20418.html
and
http://www.airforums.com/forum...stem-3592.html
and
http://www.airforums.com/forum...lush-3583.html