For several years the water heater on our motor home had a tendency to sound like it was blowing up. We took it to a shop who checked it out and said it was caused by a low battery failing to properly ignite the system. Later while we were on shore power it happened again; it was obvious at that point it was not a low battery. We had it checked again the next spring and was told it was dusty and after blowing it out it appeared to be working fine. But, two trips later it again happened. While at an RV repair shop for unrelated problem I mentioned to the RV repairman who said he would check it out. When we retuned he had media-blasted the heating tube and he stated the tube was rusty and that prevented the system from working correctly. We have made three trips with it now and it seems to be working perfect. If you are having similar problems check the tube for rust - maybe a quick media blast will fix the problem.
TNX for the added information. Every little bit helps as we are pretty much at the mercy of those we hand our problems over to. I was impressed with the technicians help in fixing what several before I'm failed to do; but then he said he has been working on RVs for the past 18 years.