Adding third roof air conditioner, fitment question.

I'm very surprised that having 27,000 btu's of cooling available you can't keep the less then 400 sq ft of motorhome cool. I would have the units checked out first before thinking about cutting into the roof. Are you sure both of your units are running at the same time when on shore power; some coaches only allow one to run when on shore power. Many folks have done a rewire to be able to run both AC's when on shore power.