I don't know why they would put styrofoam under the floor. All I know is that I feel like I'm going to go through the floor when I step in there. I think the styrofoam was used mainly because it's waterproof (yet the wood floor below it isn't) and because it's lightweight.
The '86 1550 doesn't have a full shower enclosure though. It just has a pan with a ~4" lip coming up, and this also doubles as the floor for the bathroom. Still, it may have been installed using the same techniques.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR