I also have Onan gensets, and you got me looking at the manuals
because I have no siphon breaker installed on them. The installation
book for the MDKAD 21.5KW unit calls for a siphon breaker IF the "exhaust manifold injection-port is at or below the load-waterline". Mine is about 18" above it.
It is interesting that they do not specify any particular type of siphon breaker, and none is listed in the Onan parts