I finally got the nerve to ask about possibly installing a composting toilet in my 5.0 TA, and was told there isn't enough room in the bathroom to fit one. (I was asking specifically about a Nature's Head, like the Wynns use.) It makes sense, since that bathroom smaller than the 21. So perhaps folks who REALLY want one may wish to look at 21 instead of the 5.0. And, as Donna says, your mileage may vary.