Have a look at these:
The 12 volt side of life
The 12volt Side of Life (Part 1)
The 12volt Side of Life Part 2
The water system is as follows:
There is a water tank. You posted a picture of yours. There is a 12 volt water pump. There will be a switch somewhere inside the camper to turn it on.
The water pump may have a winterization valve and syphon tube. You put the tube into a jug of antifreeze and suck antifreeze into all your pumps, pipes, and valves. The pump may be sucking air through that tube.
There will be a valve on the suction side of the pump to switch to sucking from the water tank.
There is also a "city water" fitting that you hook up using a special potable water hose with a garden hose fitting. Apparently yours is not working. Possibly the check valve inside has seized and will not open to allow water in.
You may have a hot water heater. It will hold about 6 gallons and be gas fired or gas and electric. The temperature controls may use 12 volts. The electric heater element if it has it is 115 volts.
The hot water heater will probably have between one and three bypass valves used to bypass the heater when pumping antifreeze into the pipes. The valves will be behind the heater.
The water heater may be drained. Flush the pipes first, then put the drain plug in and turn the bypass valves to put water into the heater tank. Do not turn the 115 volt water heater power on until you are sure the water heater is full of water.