I've seen the myth busters episode with the tailgate and I suppose it seems credible but I would guess with the high nose of my 5er behind me tailgate or not it's not going to make a difference.
I did hear a noise in the back once and checking the rearview mirror finally saw what was doing it. The plastic milk crate I use for a step stool for the aux fuel tank in the bed was being lifted up by wind currents. At the next rest stop I piled my heavy rubber wheel chucks in it and no more movement.
As for the aux tank, I have it so I can fill up locally in NY and make it all the way to VA before I need fuel and avoid costly PA, WV and MD. From there I can get to SC where the price is again low. The cost is secondary to the peace of mind knowing I have plenty of miles to work with on the road as I can do 5-600 miles on a full load.
2017 Lariat F-350 FX4, CC, SB, 6.7 PSD 4WD
2018 MONTY 3731FL, Residential Refer