Mine's coming up on 3 years parked on dirt / grass ... Every camp ground I've been has been dirt / grass as well. only time it's not on dirt is driving from A to B.
I guess it'll catch fire or turn to dust some day, but oh well, I'm a risk taker and I'll keep parking on dirt.
I have $$$$$ worth of automobiles parked on dirt for decades as well, I guess sometimes one or the other is on pavement while I'm at work. So far none have caught fire or turned to dust / stone.
If only my luck were that good when it came to lotto tickets.