There's doubtless some reasons for this. The two possibilities that come to my mind immediately are keeping the pipe running downhill and not having to cut into/through the frame supporting the motorhome under the floor.
If there's room to run it under the floor, and you can maintain proper slope, the only possible difficulty I can think of offhand is that you'd be more likely to have the pipe ice up in freezing weather. I don't think that would be a big problem in practice since it shouldn't generally be holding still water, and you aren't likely to have it freeze in the short time it's between the sink or shower and the holding tank. I suspect, though, that it won't be possible to maintain sufficient slope without compromising the structure of the motorhome (and possibly even if that is done).
