This is a typical water flow plan for the older trailers... The newer trailers are similar but may have alternate ways to fill the fresh water tank from the city water port. My 2008 trailer only has a manual fill port for the fresh water tank like this diagram shows.
I have one exception to the drawing and that is I have the ONE WAY VALVE built-in with the WATER PUMP. It will only suck water from the tank and will not allow water on the city water side back fill the fresh water tank when connected to city water. The other ONE WAY VALVE shown here does not allow water from the PUMP when running to back fill the city port port. I have had my ONE WAY VALVE get debris in it and when the water pump was turned it would shoot water out the city water fill port.
One suggestion on cutting through the underbelly membrane material - I would do it at a spot where I could install VELCRO or some type of sealing door so to to speak and not destroy the purpose of having the underbelly membrane.
I am also curious if you are parked like you suggest where does all your BLACK an GRAY water go. Hopefully you are capturing all of this in portable tanks and toting off somewhere for legal disposal...
Roy Ken
