well, I'm not sure I have a clear picture of what you've described, but A/S did not put grey tanks in until 1974. (73 on some models it was an option). My 73 had a tank in the middle of the trailer, but it is for fresh water. are you sure that's not what you're looking at?
the hand written note must mean that your trailer was retrofitted w/ a grey tank....but it is confusing. sounds like they may have set up some way to collect grey water in some external auxiliary tank, but I can't imagine why they would do that
the normal "no grey tank" configuration has a 3/4 inch pipe that enters the 3" dump outlet, below the valve for the black tank. waste water from the sinks and shower just flow right out into the 3" extension pipe, and on to the ground...or down the sewer hose, if it is attached. nobody cared about grey water in 1970. now, they do.
when you say "put water in the system", do you mean "run water in the sink/shower? if thats the case, and it comes out that 3/4 inch pipe and runs onto the ground, it sounds like you don't have a grey tank.
you can get an external tank that has wheels on it that you slide under the trailer to collect the grey water. when it fills, you drag it over to the dump station to empty it. the bigger one's can hook on to your truck and you can tow them over there....slowly!