We had a different event when we bought our used DP. Ours had 41K and 6 weeks later had to replace the engine, turbo, head, and injectors, to thetine of 20K. No maintenance on the air filter which had a hole in it you could stick your fist through.
But on your problem with the fire and heat could there not be some damage to wiring running from the rear of the RV to the Front?? If the RV showed no signs of this type of trouble? We had a bad surge protector that caused a fire and some weird things happened with the wiring afterwards.