There are several things that must be present or properly set before propane can flow, IN linier order.
You must have propane to flow, if the tank is dry.....
The valve must be open
Some RV's have an electric valve in the main propane line, it's powered by the explosive Gas (Propane) detector inside near the floor, if it's off, no propane flows.
The tank has an excess flow valve. Sometimes if you quickly open the valve, it will close,, However it should re-open if you sit without drawing propane for several hours (or turn off the valve wait 5 minutes and open it SLOWLY).
The solenoid valve on the water heater must work (Bad valve, bad control, bad wire, no 12 volt to solenoid).
And .. the last item.. There is a spider that loves to nest in the burner orifce, also other debris in said small hole in the brass, can block the flow of propane.
Home is where I park it!