I'm not an expert, especially on homeowner's insurance, but I have lived in a trailer before. I think that the reason that the insurance company may not want to cover the loss is because perhaps the the method that was used to thaw the pipes is NOT recommended (although, if he used a small propane torch, it is done by lots of people). The insurance company may have determined that the loss of the trailer was due to negligence and not the cause of an accident, and could really mess up reimbursement. For instance, if you have a fire due to neglected cooking, it was truly an accident, for the action you had taken would not cause a fire under normal circumstances. However, when one thaws pipes with a propane torch, the pipes are generally VERY close to the insulation, and the expected result is a fire. Trailers go up like a pile of straw. I've personally seen one burn down completely within 15 minutes - there isn't much you can do once it's on fire except get out immediately.
I've had my pipes freeze, and found that keeping heat tape plugged in all winter, although more expensive, is worth it. The only other safe method is to pour hot water over all the frozen pipes or to use a hairdryer - both very difficult in the middle of the winter, outdoors, and underneather the crawl space of a trailer.