I think the major concern would be the extra weight stressing the sewer outlet as you bounce down the road. That would not be as much of a concern if you are sitting in one place.
My main concern would be leaving a relatively expensive piece of equipment exposed to the elements unnecessarily. Another concern would be in the position the pump is when it is hooked onto the sewer outlet, it seems like water would lay right up against the impeller and seal for the whole time, which can't be that good for the seal.
