I had a 5 gallon legal gas can in the bed of our diesel truck for extra fuel for the genny this past february for when boondocking. The Galveston ferry attendant instructed me that it must be emptied and have water in it.
No problem, I topped up our '92 FLHTCU Harley in our second trailer, our 3500w Champion genny, , and gave the last gallon to the next car in line, then put a gallon of water in the can with water from the outside shower.
Trouble I see with this ferry policy, is what difference does it make if the gasoline is in whatever legal container?