There might be many reasons, one could be that the original installer wanted to be able to "share" a plug with another rig. That is, he plugs into the power pole and another unit plugs into his rig. Or, the gen-set on the bus is large enough to power another rig (charge batteries etc) and the owner wanted to have an easy way to allow another rig to plug in.
One would really have to see how it was wired to guess exactly what it might be for.
Ken