Back up Propane?

Your genset should be an Onan KY series (3600 Microquiet). That is vapor feed, so extend-a-stay type connections should work. A 5-pound bottle might not work, depending on temperature, because the generator might draw vapor faster than the smaller bottle can feed it. 20-pound and 30-pound bottles should have enough liquid surface area except at very low temperatures, but other appliances running (furnace, water heater, oven) operating at the same time might draw more than the bottle can provide. This is also the ultimate limitation on supply from your built in tank, which is horizontal to provide a much larger surface area.