My DW just got one of these washboards:
Uses that in a tub for a quick wash of an item or two if the kids fall in the mud or some such. If we have an actual load's worth, we just use the campground's laundry or a local laundromat
I would think with the size of those portable washers, you'd be better off just going with an RV washer and just skipping the dryer. Don't think you're really saving any space either way. And there's the possibility the portable's going to pull more power than an actual washer. At least it was true of a portable air conditioner I had way back when, and a portable dishwasher my parents had back when I was a teen, so it makes sense to apply the same logic to portable washing machines.