Every drain in your house drains to the same place and there are no "back valves" (except for some washers and dishwashers if your code requires it - mine doesn't). Given your toilet is lower than your sink, I wouldn't worry too much. You'll just need to figure out how to get it moved from the black tank to the grey tank. But looking at your floor plan, it seems odd that they went PAST the grey tank to tie into the black tank.
Also, from other posts I've read on this forum, I believe that there are a number of TT's out there that drain grey water to a black tank, usually related to the floor plan, as mentioned in post #6. Some of the larger TT's with 2 bathrooms have 2 black tanks and one grey, as I understand it. Usually the sink in the bathroom without a shower/tub will drain to the black tank for it's toilet.
(Note that I'm NOT defending FR; just stating some facts I've gleaned from this forum.)
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