How about suggesting to them that your Daughter needs her own room? Shame them in to it if you must.
Adding to trailmix's remark... Depending on what kind of rich people they are, you could try "I know that you work so hard to financially support daughter, and that it must be so difficult to hear the request that she get her own room. I know that must be placing a financial strain on you that you can barely afford." For certain people (not all), the implication that they can't afford something can really stick in their craw. You know your in-laws best.
Who knows, maybe with all the support they're throwing at Dad, maybe it IS a strain for them to provide a room for your daughter right now, which means that there are cracks in the facade.