Hopefully somebody with the exact model and floorplan sees this thread and can answer definitively but I remember reading somewhere that depending on the floorplan, some Thor Challengers use the front gray tank only for the washer/dryer --or washer-- to drain into.
If nobody answers, call Thor and ask for the plumbing schematics for your model and floorplan. I've heard they will gladly send it to you in a PDF file. That way you'll know for sure what is draining into what tank. It may be a good idea to ask Thor for it anyway so you can keep it with the rest of your documents.
eta: ...after doing some searching around, Thor now has an owner's portal where you should be able to pick up things like the plumbing schematics online yourself once you register.
Thor Motor Coach Owners Resources page
"Brand new to the Thor Motor Coach website is an Owner’s Resource web-based portal that gives owners access to relevant and specific information on their motorhome, including factory-installed components and equipment, owner’s manuals, electrical diagrams, plumbing schematics