Your DEF usage while towing seems to be about the same as mine. Loaded my fiver was running a little over 14.6k when I weighed it last year and stands a little shy of 13 feet tall for the main roof, plus AC solar panels etc. Gas mileage towing runs from 9 to 11 mpg per tank depending on wind, terrain etc. I burn around 1 2.5 gallon jug of DEF per thousand towing miles and less than half of that unloaded.
As for the argument about mileage with and without DEF, my 2011 RAM CTD got about 4 mpg less at 80 mph freeway speeds unloaded than the 2015 with the DEF tank does. The 2011 was a standard output engine, the 2015 is the High output which you would expect might get slightly less mileage, but it doesn't work that way. The 2015 definitely gets very noticeably better gas mileage. The downside is that Chrysler doesn't trust you with the gas pedal on the newer engine with the torque management making it sometimes exciting if you're trying to accelerate from a dead stop. The delay in getting into any engine power can be almost dangerous sometimes, but it's probably good for helping to prevent things from breaking when towing.
2015 Ram 3500 Laramie CTD, 4X4, AISIN, B&W Companion Puck Mount
2016 Heartland Bighorn 3270RS, 1kw solar with Trimetric and dual SC2030, 600 watt and 2k inverters.
