My sister and her hubby pulled a big ole Montana fifth wheel. They were never in any hurry to get anywhere and always got off the road around 4-5PM. When they unhooked and got set up they would go fill up the truck then at a regular gas/diesel station..
I guess you can never make this method work for you all the time but they seemed to have it down pretty good. They always had a diesel jerry can on hand as well...
I have also heard him say he has stopped at a WALMART or truck stop rest areas and unhooked to be able to bypass pulling the trailer through the fuel lines...
Roy Ken