With that much voltage drop I would bet your sister's batteries died an early death because of undercharging, not over charging. Good that you are adding fuses at the battery.
When filling in the fields for circuit length in the voltage drop calculator, would I just count 1 way trip since the trailer frame is the return path?
Don't see the frame being a cause for voltage drop. All connection points have been ground clean and new wire and ends used, then sealed watertight. Both ground wires at the battery and the panel to frame will be accounted for in the 1 way length