We are talking about 2 completely different things. Marine requirements have different considerations, water.
There should be no limit on the distance from the point of entry to the trailer to the breaker box. That wire is sized and protected by the fuses or breakers in the box of the campground.
Now from a practical point of view the breaker panel wants to be in the area of entry just because of the cost of the cable versus the cost of the wiring, small gauge, used within the trailer
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles