Hello Peter.
You're nearly there. If the trailers doing a constant 40MPH you will simply jump up and down on the spot feeling a bit of a dick. If its accelerating rapidly, you are denser than the air(or were last time I saw you) so you will splat on the back of the trailer.
If you were inflated with helium to the point where you float to the ceiling, You would splat on the front of the trailer if you jumped down off the ceiling while the trailer accelerates. (what a bizarre image!) Simple.
Phil