When I put mine back together, I pondered this question a while, and after wasting a bunch of time dithering over it, concluded that anything you put between the floor and the frame runs the risk of compressing or disintegrating over time and leaving a gap, movement, and squeaking in the floor. Any vapor barrier you install on the underside of the floor is just as likely to trap moisture against the plywood as it is to keep the moisture out. Instead, I put three coats of poly on the underside before installation and called it good. The R value of the radiant barrier films becomes practically nothing as you eliminate the air gap, so sandwiched between a frame rail and the floor, I can't believe that the film would provide any noticeable effect.
This is another one of those aspects that are easy to overthink/overengineer, but at the end of the day, even the most insulated floor will be worth little when your aluminum trailer is riddled with vent holes and windows which will bleed heat.