Neptune 6500
I've had my 6500 for a little over a year and had been wondering about this, too. The owner's manual (on page 5) is pretty much silent on the matter, except to say "The amount of water used will vary with each load." As you might expect, I have tried to figure this out on my own, and have done so a couple of times.
Before I say anything else, I want to emphasize that what I have observed on my machine might well be different from what someone else sees and measures on their own 5500 or 6500. Maytag made some changes in the Neptune over the years, so there are no doubt differences in the operation and performance between one year's model and another year's. My Neptune, a 2005 model, drains into a sink, so I am able to measure the water output in this way.
The "cotton/sturdy" cycle is what I use most, so I am going to report only on what I have seen on this setting. It is a pre-programmed 48 minute cycle that has one wash and three rinse phases. For the wash phase, I measured about three gallons of water were drained when nothing was in the machine. When the machine is full with a heavy load of towels, for example, the amount is usually closer to five gallons. When I do a load of clothes that I wear to work (dress shirts, slacks, etc.), the figure is about three and a half gallons.
The first two rinses, on an empty drum, held what was a little over two gallons. It is roughly this on most loads. The final rinse, when the fabric softener container is flooded and the diluted solution is added to the drum, usually has about three gallons. More water, maybe around a gallon, is added to heavier loads. This is something that the machine calculates and performs throughout the entire cycle - Maytag calls this "adaptive fill," and I have observed that different loads can and do use different amounts of water on the same setting. This is also true for other cycle selections. There is also an option for an extra, fourth, rinse which seems to use as much water as the third rinse.
I know that you asked about water amounts in an empty drum, but I wanted to give a fair description about what I have seen over the past year.
Darryl