The first problem points to air getting in the fuel line, look for leaks, esp around the fuel filter.
The water valve is indeed the likely culprit, just follow the heater pipes through the bulkhead, you will soon see it, I think they have 4 pipes going to/from it, for now I would just jam the thing open if you can, they also suffer from blown pcb's, not sure if the heater on yours is fitted with these though.
tabetha