While the boards and ladders are good for exact measurements.
If I knew the specs on the RV was 13'9", I would not go under a 14' underpass. Maybe 14'6". Having that said. Rather than climb on the roof of your RV, Just use a stiff stand out tape and measure from your tallest part to the ground. Look in your manuals at the A/C unit that came with your RV and it should have dimensions I believe and add that height and round up to the nearest foot, plus 6 inches and you should be safe if you follow the posted heights. I believe the posted heights on highway overpasses are given with a 6" allowance