I think I may have a very similar it not identical unit:
[img width= height=]http://house.albury.net.au/07feb2004/thumb.MVC-119X.JPG[/img] (Click image for larger view)
While I built a
complete microcontroller for my main genset, the Onan worked fine during construction.
Mine has a centre-off spring-return toggle switch for start/stop, so I'm guessing its similar to yours.
Something you could try (it'd be REAL cheap), would be to use a
SMALL relay for the "Stop" function, with a
LARGE electrolytic capacitor in series with its coil. I'm guessing something like a 4,700uF electro, and a relay that only takes a few milliamps coil current.
This will let the relay pull in immediately it gets the "STOP" signal, but will only stay pulled in until the capacitor charges enough to have its charge current less than the relays hold-in current.
Will draw a circuit if you need, but sure you can figure out from that!
You could possibly even use the same cap so the start relay only runs for a few seconds too, reducing waste power. As long as the delays are long enough to do the job of course!
One word of caution though: how are you turning on and off the gas supply?
I found that if the builders left the gas bottles on for a week, they'd be empty when I got to them. I guess the regulator "leaks" a little bit.