Both of the sections you refer to cover generators that are supplying loads which are all connected be Cord and Plug. Once you connect that cord to a flanged inlet and through it to a built in wiring system of a trailer how can we say that the loads are connected by cord and plug? What they are connected by is a built in wiring system which was built without electrical testing laboratory inspection, testing, and listing. Cords are Listed by testing laboratories as having been manufactured to meet certain standards. Even if I ran that cord into a trailer and plugged a Laboratory listed piece of equipment into it there is very low likelihood that the frame of the trailer would become energized. Once I plug that cord into a flanged inlet and a built in wiring system there is bound to be some voltage difference between the trailer's exterior and the earth on which it is sitting if only because of the voltage drop in the Equipment Grounding Conductor of the cord. If the Generator is not grounded then the touch potential on the exterior of the trailer could be significant and possibly even dangerous. One of the purposes or AC system Grounding is to "Stabilize the voltage to ground during normal operations."
In all my past dealings with mobile AC Electric Systems when they were supplied by power that was not being produced by equipment located on the same chassis the supply source has some sort of Grounding Electrode System. When the source of the power is on the same chassis as the receptacle outlets the chassis itself can serve in place of the earth as a voltage reference which is common to all parts of the entire electrical system and thus avoid most sources of stray voltage and dangerous touch potential. That is the entire meaning of 250.35. The only loads that will be elsewhere are those of Electrical Testing Laboratory listed luminaire or appliances. I believe that the connection of only such listed assemblies to ungrounded generators located elsewhere is the intention of 250.34 & 35.
