I've purchased 85 Octane gasoline many times on trips out west in various motorhomes (rented and owned) and also personal cars, and have never noticed any signs of detonation.
As previously stated by others, ambient air is at less pressure, so when engine compresses it the maximum pressure in cylinders isn't as high. Of course that's only at full throttle because at partial throttle pressures can be the same or higher.
What is also very important is that air in the mountains isn't likely to get as hot as at sea level. While we see 100 F in Houston or Dallas every summer, temperatures in mountains at elevation are usually much cooler. Therefore at full throttle not only are maximum pressures lower, but maximum temperature before ignition is cooler also. That's why 85 Octane has been OK for me. I also rarely use full throttle to climb mountains, if at all.
I would buy 85 Octane again in mountains unless they were having a heat wave.
Regarding generator, we don't use it often since we stay in campgrounds with hookups. And in the mountains it's cool enough that A/C isn't needed, and dash air is adequate during day if needed at all. Anyway, if we run generator at all it would be a few minutes to power microwave or make coffee, and I don't think that's going to hurt it by building up carbon. Regarding carbon I'd worry more about other variables more than 85 vs 87 Octane.
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