I wouldn't worry about a super hitch. I think you can get a WDH on up to an 18" extension which would easily handle the trailer you are talking about.
The thinking with the 815 is if pulling the trailer, you really wouldn't be staying out long enough to need the shower so the shower draining into the black tank isn't an issue. Or bypass the shower to drain into a bucket and then pour the bucket into the gray tank which is larger in the 815.
But, if you want to use the TC while the trailer is far away, something like the 915 would easily work with a longer extension. If you needed too, you could always go with a Superhitch for just feeling safer about it.