Not sure how valid this is for 1983 models but this is a typical electrical layout and water flow diagram. If you trailer is good shape you can at least start making additions if you dont have to get to something similar to this...
Of course your trailer probaly does not have the shower so don't worry about that...
This is how my POPUP is layed out showing where all my appliances and other items are located. Not a whole of room inside the popups so shouldnt be hard to locate things,
On your sink the way mine works is I fill up the fresh water tank and then I turn on a PUMP switch. The PUMP runs from 12VDC from my onboard converter or connected battery.
If you are connected to city water than you do need the pump or filled freshwater tank. The city water will provide the water at the sink with its pressure.
Roy Ken