I'm not defending FR, and I'm not thrilled with the plumbing setup you've described, BUT:
My thought was that maybe it was to allow you to put grey water into the black tank. Unfortunately, it's a 2-way street and you would also be putting black water into the grey. But I'm wondering if there was some kind of method to their madness.
All that being said, I would re-plumb it, too.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10