We've been on three cruises with DCL with my mother who has to use an ECV/scooter. The first was an Oct 2015 Western Caribbean on the Fantasy, then a July 2016 Alaskan on the Wonder, and most recently in April 2017 to the Southern Caribbean on the Magic. Each time we went to our assigned muster station as we normally would. I can't guarantee it will go this way (I don't know that it is set policy or anything), but it was essentially the same each time for us over the 3 years.
We always go down about 20-30 minutes prior to the start of the drill to avoid having to fight for an elevator. If you've cruised with an ECV before, you know people don't always give priority to ECV's, even if they were there waiting for the elevator first.
On the Fantasy and Wonder, we were outside on Deck 4 and our muster station was immediately adjacent to the door. When we stepped out on deck, a castmember greeted my mother and asked her to back up to the wall with the ECV, closest to the door. Then, the rest of us in the stateroom lined up next to her as usual - single file, tallest to shortest (tallest in the back next to the wall down to shortest in the front). At the end of the drill, a castmember walked over and released us just prior to everyone else so we could get in to an elevator. It couldn't have been any easier for us.
On the Magic, we were in Animator's Palette. When we went in, there were multiple groups spaced out around the restaurant. Our group was in the back left corner, so I immediately thought, "well, we're not getting out first here and we'll have a hard time getting an elevator". That wasn't the case; a castmember still released us just prior to everyone else, so we got an elevator quickly.
I was concerned it would be difficult with the ECV and so many people at the muster drill, but with how they handled it, it was very smooth.