Onan - Metric oil drain plug
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.
Just me, my girlfriend and our crazy Boxer Stella.
'13 GMC 1500 Sierra Denali, '14 Stealth SS1913, '08 YFZ450 with some crazy engine mods.