I camp at 9500 for the elk/deer hunt here in CO and we use a MrBuddy heater. Our hardsided popup is far from air tight but we still crack a window for some air circulation. We use a 14lb propane bottle and a hose to connect to the heater. It lasts for about 6 nites and partial days. We also use it when we camp in the summer at up to 11k feet. It says that it shouldn't work past 7, but we've never had problems with it.
We've never woken with any o2 symptoms, so I guess what we've done works. But the suggestion of a detector sounds good.
"How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?
Sherlock Holmes
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