as said before, the more time you spend in it, the more you will become "One With It's Needs".
Every trailer has it's own personality, just like your house, and you learn to read it's quirks. A good example for mine is, I can tell when I should think about filling the fresh tank. Long before it actually runs dry. My water pump changes its tune when its down to about 10 gallons. Not the pump motor itself, but the way it vibrates. It's hard to explain, it's just a sound and feeling I have learned to recognize.
Grey holding is the same. The drains will let you know by the tune change. I have no holding tanks in this one, but in my Burro
, I knew when I had only a couple gallons to go by the sound of the drain.