Here is what I have "DISCOVERED" so far.
AERORP is simply the Base Coord Reference point. In my head I call this the 0,0,0 point. IOW, Everything else is referenced from this. I am developing an OV 10 Bronco and have put the AERORP in the physical center of the A/C. Since I model my aircraft with Y axis running thru the wings, the x axis running thru the center and the Z Axis at the bottom of the wheels. This makes this point most convienent. So my AERORP is 0,0,103 , So AERORP (in reference to my 3d model) is centered forward to back on the wings, centered left side to right side and 103 inches UP from the wheels.
Now when I define the other points I simply enter offsets from this points for things like the landing gear, CG etc.
For example my CofG is 36 inches forward of the wings in the X axis and is -36,0,0 as entered in the XML file. Remember too that the CofG is entered for an empty A/C, so as you add fuel, and as fuel gets used, the CofG actually changes, just like it does in a real aircraft.
As I enter fuel tanks, landing gear etc. I just use the AERORP dummy object I created in the 3d model and measure from it to get my values. So far this seems to have worked for me.
Hopefully this will help someone else.