Kent,
I am not sure that I have all the components of this thread so forgive me if I have missed something.
One certain way that the overflow bottle on the Yanmar will overflow is if you have a failed coil inside your calorifer.
If you are making hot water using the electrical element either by shore power or the generator once the fresh water inside the tank heats up it will expand and should there be a pin hole leak in the second element that is plumbed to the engine, the fresh water will expand creating enough pressure to force the engine coolant water back through the Yanmar block and up and out of the bottle.
Why it would be happening with the Onan is a bigger mystery unless it also has it's freshwater engine coolant plumbed in a circuit through the calorifer as well?
Regards,
Trevor

Seafever
SM 425
Mexico