Cleaning it didn't fix it. I was at around 9k last weekend and same problem. Here is what it does when trying to run on propane and the problem arises when it tries to light.
It'll light, but it will keep sparking the ignitor (even though I can hear it burning). It'll continue to spark the ignitor and after a short while, it'll stop sparking it and I think it then closes the gas valve, so the pilot goes out. Repeat, repeat, quit.
I've got the wind covers off again and thinking the probe that sticks into the flame must need to be lowered or something. Its the only thing I can see that senses temperature (ie.. that would tell the unit that the flame is lit...but if this is the problem, why the flame isn't as hot at higher altitude is a bit of a mystery to me.
The 'probe' I'm talking about is item 33 on page 9 in the link. Anyone know how close to the tube it is supposed to be? Its in the flame...gets red hot after a while.
http://www2.dometic.com/e7a015ae-2585-417d-8f59-4e1c6c8be661.fodoc