Look around all sides of the fresh water tank. If the pipe you saw from underneath is a tank drain, there will be a valve very close to the tank. Also, your water pump will be in that area. That will be the place to start trouble shooting. If you have a winterization line, connected between the tank and pump, make sure that the valve is in the correct position (try both directions with the pump running and sufficient water in-the tank). If it pumps water from the tank, good, if not start with pump clean strainer, obstruction diaphragm etc. Then down the output line to fixtures.
Ken
2006 Winnebago Outlook 29B E-450.
2012 Honda CR-V AWD
Blue Ox Aventa LX tow bar and Brake Buddy Vantage.