If the trailer has a wood floor, I think the best answer would be a spray-on rigid foam that is has a low flame-spread rating (very important) and also is permeable to water vapor. The water vapor permeance is important because there is already a very >impermeable< later on the top of the floor (the vinyl flooring) and you want some way for the wood to be able to dry out so it doesn't get stay wet (water will
eventually get to it in some way).
One possible candidate is a foam like this: Tiger Foam | Spray Foam Insulation Kit | TF-200FR
Similar foams are available from other sources. Spray it on an inch thick and it would be R6, which is higher than the walls or ceiling of a Scamp
(please don't believe the R15 claims for the reflective "insulation", it is nowhere close to being that effective in these trailers). The perm rating for a 1" thick later would be between 1 and 2, which isn't as highly permeable as I'd want ideally, but it is probably good enough. I wouldn't put on more than an inch.
You'd also need to protect the cured foam from sunlight (it will break it down)--a coat of paint
will do that. Also, maybe some sort of mechanical protection against rocks and other road debris--maybe a layer of some sort of sturdy, breathable fabric.