I see this as a noise level issue. Not really a battery issue. You only boondock 3 days a year, and your trailer has a huge built in generator.
Need to decide if you will feel uncomfortable running the Onan at times because it might bother neighbors. If that is the case then bring the Ryobi. You will get a feel for how long charge times take. Worse thing you could do is ignore the built in genny. Actually want to start them every month if you have the discipline and ability. I know people who store trailers hours away so they can only cycle the generator and ACs when they go camping. Which for most is not every single month.
