Covered vs Uncovered Underbelly

as many has mentioned, I've had both. The covered was a 2000 Trail Lite. It was very aerodynamic and I do think the smooth under belly helped. Didn't like it cause would have to cut the material to fix stuff. My current TT, a '05 Trail Cruiser doesn't have enclosed underside but us very smooth under there, just a waterproof membrane glued to the laminated panels the whole trailer is made of. The grey and black tanks are wide and shallow so don't hang down much, and it has the Dexter torsion suspension so there are just two skinny "axles" going across. I have to disagree with the poster that said smooth under bellies won't help aerodynamics, this definitely does. It's part of the whole Trail Cruiser design that gives it a very aerodynamic shape and even though it weighs more than my old Tahoe, it tows much easier, as wind pressure is more of a negative than weight. A bunch of stuff hanging down on the under side creates a turbulence that prevents the trailer from easily separating from the air stream. Notice a lot of semi's have long plastic panels underneath beginning close together at the front of the trailer and widening out to just in front of the back wheels. This prevents the wind from flowing under the trailer where it would create turbulence.