[Swampdude]

Ok, with that info, it still boils down to the desired pressure. For instance, you could put a fan pointing out a hole and as soon as you turn it on, voila - negative pressure in the room. Not much, but less than ambient. So if your next question is whats a good pressure before the trailer crushes like a beer can? It sounded like you may be contemplating letting this pump run regularly to maintain this pressure over time? Also depending on the contents you may want to remove oxygen, and not necessarily draw a negative pressure. Because this isn't going to have the effect of a ziploc bag since the volume of space will not reduce hence displacing atmosphere. You may be better suited to "fill" the room with nitrogen, a great purging resource that displaces oxygen.
Besides all that, lets say you wanted to get the trailer to -20psi in 1 minute, and you already gave the volume of 40'x8'x8.5'.
2 options from here are:
1. you oversize the pump and use a pressure regulator to regulate with a +/- of acceptable range - which would have a big motor and little pump
2. you size the pump to run constant -20psi - which would have a big pump little motor

I assume you'd want option 1 - for efficiency?