Unlike your RV's engine, the Onan generator's engine is not wet sleeved. Therefore it does not need any SCAs. Just about anything will do as far as antifreeze is concerned. And yes, you are right, there is no filter on the Onan.
Some of the older Onans came with basic "green stuff" heavy duty antifreeze. My '04 Bus came with that. My '07 Bus came with Fleetguard's ES Compleat (blue stuff) antifreeze.
I have replaced it with and OAT coolant. The OAT coolants are extended life and feature Organic Acid Technology. In a Cummins engine this negates the need to check SCA levels and the coolant is good for 6 years with an extender added at 3 years. It's great stuff.
Your Onan doesn't "need" OAT coolant or anything with SCAs. However, it will benefit from it. Fior one thing you won't need two different coolants because the same thing that's in the Cummins can be in the Onan. Secondly, OAT coolants do not attack aluminum like non-OAT coolants do. There's lots of aluminum in your Onan so you'll gain some benefit there.
For more details check out RVcruzer.com - Coolants for HD Diesel Engines in RVs