every truck will vary and winter fuel can effect the mpg as well. Driving style, distance driven daily, stop and go traffic, wheels, lift and so on. The more miles you get on her the better it will get, but like I said, some trucks sip, some drink and some guzzle!
My LLY, 2004.5 I see 14.5 in town and on the freeway @ 70mph, my sweet spot, I can pull 18.6 to 19.4 (best) hand calculated.
At 50 mph you are still in 5th aren't you? Up your speed to 57-58 and you may see a slight mileage increase. I always aim for 67 ish. Starting to forget stuff, haven't seen my truck since Christmas :rof
I truthfully have always had one mentality about these trucks... if mileage has the possibility of being an issue, get something else.
My best guess is your regen is occuring fairly often. Diesels of today are not made for your style of driving, they are truly made to be let loose on the highway. My old commute to work was 75 miles a day round trip in the exact same truck as your driving but mostly highway, over the year an a half I made that commute, 90% of the time I hand calculated my weekly fill up to be over 19 mpg.
On average I worked 6 days, Tuesday - sunday, fueled every sunday night after work, and would typically regen every 8th day if I didn't go out and horse around.
Is that your tank average? On short commutes like that you will have extra idle time plus decreased efficiency till its at full operating temp. Tdavis nailed it on the head when he said these trucks aren't meant for your style of driving. The majority of engine wear comes before the engine is warm so you can either live with a crappy mpg avg due to idling which can also junk up the egr and cause problems, cause excess wear or buy a commuter car. Your best bet is buy a crappy civic for the commute