In very general terms, it seems that when it comes to rotting floors the area just behind/beneath the refrigerator is a rot prone area. For those older trailers that have a hole in the subfloor for air to circulate up past the back of the fridge, I would assume the rot is due, at least in part, to water splashing up from beneath. There is also usually an access hatch in the exterior wall just behind every fridge, and when the seals dry out and crack, these are as prone to leak as any window. You may alos have leakage from the ceiling "chimney" penetration, that gets funneled down to the floor on the backside of the fridge.
So short answer to your question, I suppose is "all of the above, and then some." Condensate could be a culprit as well, but I would guess it is a minimal contribution. I have a small freezer in my garage (in Houston, TX), and have never observed condensate on the outside of the unit, even on the most humid days.