I would wrap some black tape around the "plug" so you don't scratch it. Then take a pair of pliers or channel locks and gently rock the plug back and forth to see if it will break loose. It's likely just held there by friction or some small amount of foreign material and should come loose with a little bit of "friendly persuasion". Good luck!!
Days camping (2015)----88 days
Days camping (2016) ---110 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi
30 years RV'ing
12 different RV's