Look for an owner's manual on the front page of this forum....scroll down to the bottom of the page looking for the red bar that says "LIBRARY". There's a button under there for owner's manuals. Look through those and if you don't find a manual for 1987, just look for a manual from a year close to '87. Things didn't change that much from year to year.
You have 2 water inlets. The one behind the locked door will allow you to fill up the water storage tank in your RV. Once you fill it up there's a 12 volt motor that should provide water pressure to all the water faucets, toilet, shower, etc.
The other water inlet allows you to use "city water" from the campground.
So, you generally use one or the other......water stored on-board, or city water.
I use the city water if possible to avoid hauling the extra weight of a full water tank.
Before using the stored water tank, you might disinfect it by running some water with a dash of plain household bleach through the system, then rinse twice.