I don't have a Carnival yet, but I have it's predecessor, the Sedona. My '16 Sedona tows pretty well. My previous tow vehicle was an '07 Subaru Tribeca, and the Sedona tows better than that, likely due to the longer wheelbase.
I have an enclosed 6' tall 5x10 utility trailer. When loaded to the gills with 3,000 lbs (or slightly more), my mileage drops to 12-14 at 70-75 mph (from 24-26 when not towing). I think most of that drop is due to wind resistance from the trailer. Slower speeds get better mpg, but I don't have the patience to drive slow.
The Carnival has an additional 14 hp and 9 ft-lbs of torque over mine, so I would guess it would take a little less of a hit to mpg.