Sounds frustrating.. If Butane torch heat on thermocouple won't trigger gas to flow, then clearly something else is wrong... Among options could be inadequate gas pressure at tank burner, or possibly link to thermostat, which tells burner control unit that water need to be heated.. Thermostat is separate unit that clips to side of tank, and has wire back to controller... Not sure I'd agree that unit needs to be completely replaced, but you may have reached point where RV service shop or tech is needed to finally diagnose and fix problem.. In my mind, water heaters are "dead" when tank springs a leak or burner unit/chamber gets so corroded or rusty the integrity of burner unit no longer safe..
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...