I don't think that you must have the ATF level check by a dealer before flat towing. There wouldn't be any reason to include all the pictures and instructions of how check the level, adjust, etc. and the caution about using non-approved ATF.
As I stated in my post - the supplemental manual states that it must be verified by a dealer. And I'm sure that Ford would love to use that to deny putting in a new transmission in any vehicle that burns up who hasn't done that. Otherwise, they wouldn't bother to put that in the manual.