Depending on how much work you like to do, you can buy 2 part spray foam kits and do it yourself. The stuff sticks to anything - including eye glasses, hair, skin, etc - I know this from experience! I did our attic with the 2 part spray foam. It is not a pleasant job in confined spaces, but it would not be too horrible outdoors. The hard part would be getting the trailer up high enough to have some work space. You might also call some spray foam contractors and ask what they would charge to do it. Some have special purpose trucks and buy the components in 55 gallon drums - they have the best pricing. The heating pads I don't know about but they would require some wiring and drilling at least one hole in the trailer.
Eric (and Mary who is in no way responsible for anything stupid I post)