Probably what you are seeing is local requirements above those of Art. 550 and maybe Art. 552 (Park trailers). Most of the counties here require a pole within a few feet of the mobile home with the meter and a panelboard mounted on it with at least a 100 amp breaker to serve as a disconnect outside in case of fire. Schedule 40 PVC was Ok a few years ago when I installed one. That may have changed recently. If I had to install another, I would check the local requirements.
Park trailers are limited to 400 sq. ft. Thats a 10X40 mobile home. Many travel trailers are that size and larger set up in parks. Local ordinances would probably dictate more stringent requirements than in the NEC.
