Jon makes the important point. While this may still be legitimate for the discussion's sake, for mud or sand, this feature only occasionally works, for other reasons. The weight of these coaches more likely will help getting more stuck as holes are dug by the drive axle wheels. Yes, very handy that one or two times. But Newells get stuck too, as drivers may think they can just hold the throttle on and wait. Then the shovels come out, if it's possible to dig under the coach bottom.
Perhaps more to the point, it's unlikely, if not out of the question, to convert. It means replacement of the drive axle with one that has a locking differential, plus the ancillary equipment that goes with it. Huge undertaking probably more expensive than an engine rebuild or replacement/exchange, if anyone contemplated it.
Anyway, that's pretty much the story.