I am going to guess that you have an NT series heater, the valves need to be tested individually, when moisture gets in one valve it often corrodes that one shut, (water reacts with the aluminum body) it does require pressure to operate them as well properly. (10 inches of water column)
If the doc center is up...
I sent the complete service manual there for all to use, it does cover the whole testing protocol, if the Doc center still has the flu I can send the manual to your email.
P.M. (personal message function on this site)
me the model number please if you request this, post your email to me in the p.m.
and I will email you the pdf.
I also have extra valves all over the shop.
Happy Camping, Safe Trails.