Jim, (Olympia, WA)
There were two types of ECOs used in those older pilot model RV water heaters; one was a bi-metal thermal switch of sorts that should eventually reset itself once the water temperature cools. The other type was electronic by design and will not reset. It's a one-time safety and once it's tripped, there is no way to regain continuity through the device. Chances are the whole control assembly will have to be replaced if the ECO is the culprit.
As old as that water heater apparently is, however, I would consider replacing the entire unit and obtaining a new warranty, etc. The newer models are much more efficient and offer the best in the latest technology. You certainly would not want to invest in a costly repair only to have the inner tank rupture in the coming months. I'd vote for a new unit if it is the main control valve at fault.