I have never had an RV that didn't have a drain valve for the fresh water tank.
If the tank is exposed under the trailer, there could be just a plug in a corner of the tank that needs to be removed to drain it.
On my new 2019 Forest River 2511S it has an enclosed bottom, so no tanks are exposed, but there is a simple drain valve on it that you have to reach under the side on to get to it. It also has electric 'tank heaters' on it. Not sure if it includes the fresh water tank, but I think it does.. I really won't have any need for it, as I drain my tanks and don't camp in freezing weather... :)
Good luck!
Mitch
