I think most would say 9-12 average for 2017 models that I’ve seen posted here depending on how they drive and what terrain they travel in. That is, is it flat at low elevation or regular higher mountains? Do they never drive over 60 or do they routinely drive up to 75? Is the TC a tall one or short one? Is the truck towing a heavy trailer or is it without trailer all of the time?
One other thought, most people say they got about the same MPG with fifth wheels as larger TCs due to the wind resistance being the biggest factor. Assuming you agree with that reasoning, you could look for what MPG fifth wheel owners are getting and have a good approximation.
In any case, if you search the archive here, you can find a lot more numbers.