Originally Posted by Wingerdinger
I have never seen or heard about an RV where your city water and fresh water ports aren't near each other. If you have connected city water, your fresh water port is nearby. Most of the time, they are on the driver's side, but I've seen a few older ones on the back.
Edit: apparently, this information is not accurate, and I miss-read the OPs question. The fresh water tank will have a drain on it, hanging below the belly. Generally, there is three water drains, two are 'low point' drains, near the water heater, so you don't want to mistake those. On mine and a few friends, the low point drain hoses are blue and red, the fresh water drain hose is white.
I hope that information is more helpful than my last.
Perhaps you could tell us why you need to locate the actual tank?