Those thermostats are tiny and there is (2) one on top of each head under the small housing that the outlet water hose is attached. There is also a " T " in the line where the water goes down from the pump housing and enters the bottom on the block on one side and then runs over to a 90 degree fitting where it goes into the block feed on the opposite cylinder. From working on two different ones recently I would say if you feel like you need to flush it out, you can do it with out pulling the thermostats as there is a .250 bypass but if you do pull them out the flow will be much higher. There does tend to be a good deal of sediment that accumulates in the bottom of the heads and block that a good back flush would assist in removing every once in a while anyway. I would pull the pump discharge off the T fitting and start there first and you will know iff the impellor is the issue or not. I am building 1 good onan gen. from 2 bad ones right now and I am going to reverse the water flow with the thermostats out while bench testing just for this reason and then put it back to original before I install it.

He is absolutely correct, I know to do it but often overlook the simple things which are normally the answer.

The water passages in the onan heads are quite narrow and prone to blockage if raw water cooled.
These are the Achiles heel of an otherwise great genset.
The heads are relatively easy to remove and clear.
If you have zebra mussles up there in MN you might check the intake hose as well.

I have a Kohler and was quite surprised about ten yrs ago to be shown the clogged cast iron raw water piping around the engine.

FOUND THE problem.
Before removing the exhaust manifold there was a a short hose to a brass fitting that was almost completely clogged.

No discharges water according to specs.
Thanks for all help.
Garyh