Your Plumbing is going to be broken down into two groups. Fresh water supply & storage, and Waste water storage & disposal.
Both groups are further divided into two more groups. Fresh Water can be carried in a fresh water storage tank within your trailer to provide water by gravity, a manual hand pump or a 12VDC RV type motor driven pressure pump. The other option is what is referred to as a “City Water” connection. A pressurized connection via a hose to campground water or other type of hose bib.
Waste water is normally plumbed separately to either a “Gray Water” or “Black Water” storage tank mounted somewhere on or in your trailer. The gray tank collects all the sink and shower soapy water. The black tank collects the toilet waste.
You want to locate your fresh storage tank to be balanced over your trailer axle as close as possible. Also on the most convenient side as to make pipe runs as short as possible. In almost all cases it should be on the drivers side. The reason for this has to be how campgrounds are laid out. At least in USA. In almost all cases when you back your rig in, or pull through, the utilities will be on the drivers side. Fill tubes, Electrical. Water Hookup, Sewer connections and Cable TV connections.
With that in mind I mounted my fresh water storage over my axle on the passenger side. That dictated part of my design. I hide it in the bench seat area of my dinette, on that side of the trailer. My shower and toilet area went forward of that. Which then dictated where my gray and black waste tanks went below the sub floor, between metal supports of the trailer frame.
You need to also locate your “Hot Water Heater” if you so choose to install one. These are typically installed on the drivers side as well. As it your LP Furnace.
Once these vital component have their place, partitions and cabinets can be designed and things framed in place.
Before applying any insulation or interior wall skins run your 110VAC & 12VDC wiring, water lines, drains, and LP lines. Pressure test both your water supply lines, as well as drain plumbing before proceeding. Fully fault check any wiring. I personally have all systems up an running before starting to finish the interior off.