Enclosing the underbelly is not about towing. It is to insulate the underbelly (to a degree). A bigger issue with winter camping where it helps stop the tanks from freezing but even in summer it will help a bit.
We like the slide toppers as it keeps rain and debris off the top. If you camp in the woods without a topper, you have to sweep the branches and leaves off before pulling the slide in. With the slide topper, you generally don't have to do anything. Plus it helps with cooling as the top of the slide is shaded.
