Honestly the coroplast covering is probably more likely to be an attraction is it provides a sheltered place to live.
That said my impression on my 2604WS is that the covering between the frame rails is fairly well sealed up. They do cut/hack holes for plumbing egress but they they shoot them up with black expanding foam.
The open sides are just the plastic covering over the trailer deck. The deck is about a 1/4" plywood sheet on top and on bottom with about 1 1/2" foam core. After the tanks and some wiring, this is the first part put on the trailer during construction.
They put a lot less holes in the bottom of the deck than they could. As an example the water hoses from the water pump and wires do not pass under nor over that deck, they actually pass *through* the deck in between the skins.
When they do put in a hole they are pretty crude about it though but they must get a great deal on black expanding foam because they use enough of it.