Shasta 18fq water heater problem
I had a Shasta 20RL TT and I had trouble every so often. Here are a couple of things that may help. What size fresh water hose are you using, I changed mine to all 5/8" hose. It did sound like you are relying on the water tank, If that's the case you will need to keep that full, or you will add more air into the system. I know 5picker mentioned this, but when your filling especially if the system was drained, you may need open the temp safety valve and fill it a little slower (or the igniter will get wet and you have another problem). Once water comes out from the relief valve, then close it and have someone open a hot & cold water sink valve until you have water. This should solve your air lock problem and it should only happen when the system is drained.
I hope this helps, now that I wrote my first book!