I believe you will find absulotely no gain with or without the tail gate. I think most people use a 5th wheel tail gate for convenience, or as a replacement for the one they screewed up.

 

I don't think you will see any increase due to the tailgate, but you will increase the milage a lot by reducing the speed even just a little. I travel at 60 mph and get around 13 mpg towing a 10K fiver. There are lots of variables such as grade, wind, if your truck is well broken in, etc, however there are lost of postings about towing milage at various speeds. It seems pretty clear that the faster you go the lower the milage.

 

this is kind of off the subject a little but last week i hauld about 2 ton of hay in the back of my truck and did not clean the bed out. Well hooked up to the 5vr and headed down the road no big deal! tell i hit about 65mph and the tornado winds kicked in and the hay went eeeeeeeeeeeeeeeeeevery were in side the cab on the car behind me up the side of the trailer it was a huge mess but i laughed my butt off for miles