Hi Angier......I also have a 66 Safari and love her!!
The city water hook up is inside the rear access door. I re-plumbed mine so that it hooks up on the back side of the trailer and I don't have to feed the hose into the rear compartment. Before that, I fed the hose up through a pipe that comes out the left rear bottom of the trailer (where the power cord drops out). Look there and let me know if you don't locate it.
Your water tank is under the dining table and runs side to side at the front of the coach. There, generally, is a wooden cover built over it and the pump should be on the left side under the seat towards the front of the trailer.
I am not sure how to tell you to open the cap from the outside. Mine opened without any problems. Be careful with it though...you don't want to damage it. If I remember correctly, Inland RV may have replacement caps if you do end up damaging it.
I have done quite a few things to my Safari and can answer questions for you, so, keep in touch and good luck.
Got an pics of yours? We all love to see inside and outside pics!!