Mine is 60 gallons and uses every bit of space between the frame rails and cross members. It hangs down a little and so do the brackets. No filler neck, just a small hole in the floor for the filler and cap with a small locking access door on the side of the trailer. Just have to make sure I don't park over the filler if I need to fill it up.
If you mount near the axles I don't see why you can't hang down below the frame/members a couple inches.
My generator has it's own fuel tank in the left front corner of the trailer that hangs down a tiny bit and it's never been scraped or touched by anything and neither has my fuel station tank and my trailer's 20 years old.