I've been using the Mt. Lp. for hiking, cross-country skiing, etc. since I was a kid. There are no hook-ups along Mountain Loop Highway. It's all dry camping. It is very remote through rugged mountains with hardly any elevation gain.
The only way in from the south is through Lake Stevens off WA-9 to Granite Falls on WA-92. From the north via Darrington on WA-530 from either Arlington or Rockport on WA-20.
From Granite Falls to Barlow Pass it is FS campgrounds along the Stillaguamish River. From Barlow Pass to Darrington camping is pretty much just primitive pull-offs along the Sauk River better suited to truck campers and tenters. Hiking/backpacking access into the Glacier Peak Wilderness is in this "Darrington" section of the Hwy.
Personally, I would not take my rig into the Mt. Lp. Hwy. A small trailer with a generator - yes. Longer than 23' - no.