The Park manager where I used to work had one. The front of the bus he made into a camper and the back was the trailer, He drove it all over, He had to make sure there was a place to unload though, a lot of campgrounds and parks used to have mounds of soil with a railroad tie wall built into the mound on one side, and it was used as a ramp.
He built one at the park where we worked. I think it is still there.
The ramps are not around much any more, I guess since trailers are built differently, they are either low to the ground or have their own ramp.