First off... what a great site. THis is a wealth of info and have found several answers to things i have been wondering about for a while.What my question is, I want to put an access opening in the fresh water tank. I see the fiberglass cover (or whatever it's made from) that is sealed and screwed down... Is this the actual top of the tank, or is there a tank underneath this cover.If i cut through this to put in one of those 6" dia inspection access plates will it cause me a problem with the preasurized water system. I don't want to find i can't keep water preasure.