Cummins onan oil change

If this really is a "brand new" RV, there should be an owners manual for the generator in there somewhere. If not, see your dealer.

I think both my Onan's have had a 150 hour service interval.

When I had my ONAN, I just changed the oil every spring when I de-winterized. Of course, we probably never hit 100 hours of use per year.

I never hit 100 hours per year so I change yearly...usually early spring time when I am also taking it out of its winter hibernation. There is no oil filter, just a simple drain-and-replace.

I believe the owners manual will advise as to the correct Oil Change interval. Ours is a 5500 and the interval is at least 150 hours. Make sure you follow the recommended interval and use the recommended oil grade. Don't forget to change the filter each time as well. Our oil grade is 15W40 as recommended for the North American climate.

That sounds about right - first oil change is usually 20-50 hours depending on model and then about 100 hours or at least annually.

Rather than tell you what I do, here is the answer to your question.
From the Cummins RV generator handbook.
“You should change the engine crankcase oil after the first 20 hours of operation, and every 100 to 150 hours thereafter. “
https://power.cummins.com/sites/default/files/literature/rv/F-1123-EN.pdf