I believe the operating manual is referring to an adjustment to the air/propane ratio to compensate for altitude. I tried this question on the Good Sam forum and got about four pages of responses. Most respondees, that have actually gone to the altitudes I'm talking about, say they had no problem even on long stays. One reported that the flame burned a little yellow, which is as I would expect, but they saw no long term damage. Unless I hear otherwise, I'll probably just go ahead without adjusting the mixture and see what happens. After all, I think they have hot showers at the campground.
This should teach me not to read manuals.
By the way, I'm also old enough to remember how to adjust a Holly double pumper and overhaul a Q-jet.
2009 FF X-189 FDS
2011 Chevrolet Z-71 LTZ 4X4 p/u.
Reese WD hitch
Patient and understanding wife