I don't think there's any issue with running 15w30, especially in the summertime. It's thicker starting out but when hot, both oils are the same viscosity. It may take a little longer to warm up. If cold weather camping I would lean towards the 5w, but onboard generator in the summer I just don't see an issue with 15w. You can always split the difference and use 10w.
Essentially, the lower the first number the thinner it is when cool.
I was running Honda 2000 Companions until we recently upgraded the camper. Now we have the onboard 4000 Onan and I am about to change the oil for the first 20hour service. I was leaning towards 10w40 synthetic.
Someone correct me if I'm wrong.
2015 XLR Nitro 30FQSL
2015 F250 King Ranch