On my Firestorm the manual states that the water heater will not work if a window is open above the exhaust port. I take it you have removed the cover from the vent as this will also stop the heater from working.
Hi Jim I posted a similar question some time ago & have yet to action the recommended replies as the van is still in storage but the most likely culprit I feel in my situation is that the gas tap handle has been "murphied" - that is I think the gas distribution manifold which has 4 taps on my Bailey - one each for the fridge,stove & cooktop, water heater & cabin heater is rigged with two taps that are open when the handles are across the pipe and with the other 2 aligned with the pipe.
Alarming but possible!
I had never considered that as a possibility probably because I'm invoved in the aviation industry where such options are carefully designed out before ever reaching the factory floor as things like that can kill people in my game. I'd be interested to see how you get on with your trouble shooting. My van is new & I cannot get either the fridge or the water heater to run on the gas. The cooker & cabin heater run just fine.