There are also the Olympian Wave heaters (Wave 3, Wave 6, etc), which will work at higher altitudes.
Is it safe? Nothing is perfect but I sleep with a Wave 3. I also over-vent, which may defeat the purpose of a heater, but that's the only heater I've got. I think you just need to pay close attention to your venting, and how things might change when you're asleep -- will snow pile up and cover your vent, etc?
If your CO detector has a numeric display that shows how much CO is present, I've found it helpful to experiment with different venting setups and watch what happens on that display (ideally when I'm not in the camper). My understanding -- which could be wrong, so verify for yourself since it's your life -- is that the Wave produces CO when it malfunctions, usually as a result of low oxygen. If that's true, then any number above zero on my CO detector tells me that the current venting setup is not adequate. There's not enough CO in there to kill me or set off the alarm, and there's still enough oxygen to stay alive... but things are headed in the wrong direction if there's any CO at all.
Edited to add: I also realize that some people refuse to sleep with an unvented heater, and I don't think there's anything wrong with that. People have died and you have to be very careful.