Let me get this straight. You're going to sleep in 18 F. temps with a WM 30 bag?
My WM Megalite is a 30 F. bag and barely good to 25 F. even W/long johns.

You need a serious insulating layer over yer polar weight long johns, like 300 weight polartec pile pants and jacket.

I slept OK in my Megalite (a larger girth WM 30 F. bag) at 20 F. by wearing a Thermolite Micro poly fiber insulated pants and jacket but over midweight poly long johns. Beause the Megalite does have that larger girth wearing bulky clothing did not compress the down on top of me.

That was this October at Arapaho Pass, Colorado at 11.300 ft. and in a tent.