I found other threads on this issue useless. You can't fill it via the coolant cap, and then "burp" it. Why? because the hose from the coolant reservoir goes into the bottom of the radiator, creating an air lock. The water pump which is up high on the engine cant pump air.
Like all other vehicles I have ever seen, it must be filled by removing the radiator cap, which is concealed behind the cover above the start/stop switch.
Hint: there is nothing inside the generator needing access. No need to remove the front green cover.
Consult the owners manual for correct antifreeze type. Mine calls for 50/50 mix of either green or yellow with rust inhibitors. Mine takes slightly more than a gallon. Mine is a circa 1997 Onan Quiet Diesel 7500.
I suggest wearing leather work gloves so you don't accidentally get hurt if the stuff gets too hot.
When working on it try not to tap the start switch. It wont start if you just tap it, just be aware in might happen so you don't freak out.
Here are the steps:
1. First, put duct tape over the breaker as seen in the photo below so coolant cannot get into it. This will likely damage or break the breaker.
2. Remove the bolt holding the cover on. Slide the cover way down until it can be swung up. The radiator cap is trapped in a holder with a fuse on either side. You may need to remove one of the fuse holders from the assembly to get the radiator cap assembly out. The radiator cap assembly has to be rotated clockwise in the assembly until its tab clears the slot at the top of the assembly. Then you can slide it out of the assembly, and pusht the cover/assembly out of the way to the Left. Then you can remove the cap.
3. remove the drain cap underneath and let the old stuff drain out. This probably will not drain the stuff in the overflow. You might need to suck that out with a piece of tube and a wet vac.
4. Replace the drain cap and tighten firmly.
5. See the second photo showing the fill funnel assembly clipped to the door up high. Slowly pour new stuff in but not too fast because it will burp. When it is full it will overflow.
6. Remove the fill hose and put the radiator cap back on. If you do not put the cap on it will squirt stuff out one of the little hoses teed onto the assembly and you will get a bath.
7. Turn on the AC so the engine runs at high rpm after you start it.
8. Start the gen and let it run for about two minutes so the stuff does not get hot. The gen should not shut off due to overheating.
9. Remove the radiator cap and see if it needs more stuff. Replace the cap and run it again for a couple of minutes.
10. Fill the reservoir. Mine does not have a dipstick, I used a popsicle stick. I put a bit extry in it so it can suck it down if it burps a bit more air later. I suggest rechecking the level for a few uses after it has cooled down to make sure it is still full.
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