The fresh and gray water tanks are 8 gallons each. This can last you anywhere from a few days to a couple of weeks or more - depending on how conservative you are, and also on whether you are getting your hot water from the sink (by boiling it) or not.
We have the optional Coleman water heater. For the first year we never used it because my wife didn't see the point - after all if you needed hot water you could just boil it. But, after I set it up once and she got used to the convenience of instant hot water, it has become a standard part of our equipment list. We connect the Coleman to the trailer's propane (rather than using 1 gallon LP bottles), but we don't use the trailer's water supply.
Instead, I fill up the Coleman 5-gallon container at the campsite. Since this needs refilled about every other day, it means the fresh water tank will last MUCH longer. (I would guess that 80% to 90% of water usage while camping is for cleaning.)
Also, since your hot water supply gets to sit out in the sun, the Coleman is able to heat it much hotter.
We love the Coleman, but it doesn't go everywhere with us. Along with the side tent, 10 x 10 pop-up, lounge chairs, etc, I would only use it when we will be camped in the same place for 2 or more nights. Road trips have a completely different set of gear...