Every online source I've found indicates that the Onan 4K (and similar Microquiets) oil drain plug is a 1/4" NPT, with a 9/16th head.
My drain plug is a
M12x1.5. That's right. Metric.
Not sure if this is a recent change by Onan (trailer is a 2014), but should you find your self needing a replacement make sure you know what you've got under there.