That white stuff is probably pipe dope that was applied to keep the plug from leaking. The least drastic approach to cleaning it is a wire brush made for cleaning female copper pipe fittings prior to soldering. They are not very expensive and can be found in any hardware store.
Is your plug the type with an attached sacrificial anode? Yes if suburban, no if Atwood. If so, the problem may be that they are a little difficult to put in straight. Usually they want to tilt down. I have to hold mine with a large channel lock plier in order to get it started.
2011 Silverado 2500HD Duramax, 4x4, crew cab, long bed
Palomino Puma 253-FBS, 27' 5th wheel
1994 19' Class B on Chevy chassis