This may be a bit of an over-simplification; and perhaps I'm incorrect, but I'd say that one of these combo units for an RV is ideal: space is very important, weight is very important, and they generally receive a LOT less use than one in a house or apartment.
Let's say the lifespan of a typical washer is 12 years. (Not sure what mine is; I can do repairs, so conceivably, mine can last the rest of my life.) But anyway, 12 years. Happy with that? Now consider this combo unit - it too lasts 12 years. Except, 6 of those years are as a washer, and 6 of those years are as a dryer, sentences to be served concurrently. I would think a lot of parts - e.g., the main motor, are used for both the wash and dry cycles. Hence, they have double the wear of a regular washer.
Now, this wouldn't be bad if the combo unit was the same cost as a single washer or dryer, but the cost is about double the cost. So, there's no point, except for space savings.
I'm also incredibly leery about a dryer that's "ventless." Where does the water go that's removed from the clothing? Either you're using a lot of extra energy to pull that water out of the air as the clothes dry, else you're putting that vapor into the house. In the winter, in certain types of structures, with certain types of heat, it might be desirable to increase the humidity. But... not in general.