If the insurance company thought there was any hope of finding the cause, and would be able to prove it, they would. That would give them an opportunity to get what they pay you back from the responsible party. They do have to weigh the probability of being able to do that against the cost of doing so (hiring fire origin specialists and engineers to pinpoint and document the cause - can be very expensive). Now, whether or not you'd ever know they were doing this, or share the results with you, that's another matter.
Jay & Peggy Monroe with Dolly
Can't take it with you - don't plan on leaving any behind
2016 Newmar London Aire 4553, Spartan chassis
Wrangler Unlimited toad