Hi Lisa ~
Welcome to the forums and vintage Airstream ownership. I went through many of the same questions when I brought mine home in Sept. of '01. Let's start with your first question. The fresh water tank should have a fill tube that is outside of the trailer. Where it is located will depend on where the tank is located. Most cases it is on the front curbside (side of entry door) and should be about 1 1/2" in diameter, have a slight downward angle as it enters the trailer and have a screw on cap. This would indicate the fresh water tank to be under the couch or dinette, whichever your trailer has, under the front window. In some cases, the tank was placed under a bed and this would put the fill tube on either side of the trailer depending on the bed's location.
The trailer probably won't have a toilet water (black water) tank given its year of manufacture unless a prior owner (PO) installed one. It would be directly under the toilet so if your toilet is on a raised portion of the floor then it just may be so. Press down on the toilet foot petal and look inside there. It may not be pleasant but you may just see the ground when doing so. This would also indicate no black water tank.
My trailer wasn't set up for a battery so I can't help you with the charging system.
The electrical panel with the two switches is more than likely breakers for the 110 volt house current (shore power). If this is a Bulldog brand panel, the switches should have the amerage rating on the side of the switch. Should have either a couple of 20's or a 20 and a 30.
As for your wires black taped together, I'll bet they are just above the floor and on the curved wall that is the front curbside corner and I'll bet even money they have light blue insulation on them. If so, then one of them should go to the overhead 12 volt
lights and those were powered through the tow vehicle through a special power recepticle on the outside of this wall which was connected through a different cord than the umbilical cord to the tow vehicle (TV).
Pictures of all of these areas would greatly help answer your questions. Again welcome and I'm sure you are going to have a great time exploring and getting to know your trailer.