Yep, almost for sure that the 3.6 is an LP Onan... same generator as the 4.0, just derated for the lower-energy fuel. Run time may be an issue, depending on how you use the RV. If you're planning a lot or travel in warm weather with passengers in the back, the LP 3.6 may have 'short-legs' between fuelings. Every time I've bought LP, it's a hassle to some degree; the recent price spike makes me glad I don't use it for the generator. But if you seldom put long runs on the generator, it's not a big deal.
Jim, "I don't believe in astrology. We Leos are known to be skeptics."