I have one question/comment. After you "flushed" the black tank, or attempted to, did you not drain it again, and didn't notice that nothing came out? A lot of folks have a clear elbow so they can watch the tanks drain, and know when they are empty. I usually just keep watching the hose, or feeling it, to determine if water is coming out.
I do sympathize with you over your problems, and agree should never have happened.
One other lesson learned for us all, drain your fresh tank every once in a while, especially before winter, even if you never put any water in it, some one else might have. Also eliminates carrying unwanted/unneeded weight.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT