I was taking aerodynamics in my younger years and the truth is that when it comes to air drag, the mathematics don't work.
The subject is so tricky that only wind-tunnels with smoke nozzles and hours spend on observation can answer the question with not much doubt.
Coming to trailer down or up forces, the rule is that air going with higher speed is having low density, than air going with low speed.
So my first guess would be that air deflected on the roof of the trailer goes faster than air pushed under suspension.
Meaning the trailer will get some aerodynamic lifting.
Still 50 or even 100lb of lifting, or downforce is nothing to trailer weight.
Nice spending of time do.