UPDATE !
Re-installed the ORIGINAL thermostatic switches. I confirmed (12V test light) that the switches were both closed when the tank was filled with cold water. It took a couple of attempts for the burner to start (air in the gas line), but it did. It also shut down "properly".
Waited a few minutes and the turned on the hot water tap. Confirmed 12V in to the the main thermostatic switch. Could not confirm that that the ECO switch was closed (no DMM handy), but the darn thing fired up !
We will see what happens after a weekend of camping !
