I'm not familiar with this sort of problem, but I'm sure you need to have excellent contact between the heating element and the boiler section. I'm not sure you can get that with jb-weld bonding a steel tube to the existing metal. Can you possibly strap a piece of metal over the "hole' and clamp it in place with screw type clamps. Id I understand you problem, this would allow you to attempt to use the lp gas heater to see if the unit is operational. be careful of leaking exhaust fumes while you try this.
Jim and Lynda, (Sophie, Jake, attack trained killer Shi-Tzus :-))
2003 Fleetwood Expedition 38N 2005 Saturn Vue