My 2¢
(Disclaimer: I am not a medical doctor nor HVAC expert)
The greatest danger of harm/death in this situation is from carbon monoxide build-up, dangerous because CO has no odor and it puts you to sleep before you die...so you don't know it's happening. So, it's important that you have a working CO detector -- standard in campers, but it needs to be working...and they have expiration dates on them.
I don't have a Buddy, but I have a Wave catalytic heater -- exact same issues in general, but slightly different features, apparently (mine doesn't have a low-oxygen switch, for example). Here's an example of the value of a working CO detector -- my own experience:
On my first trip with the Wave (late October at 8500' in the White Mts of California), one morning I got up before dawn to take photos...but it was heavy-overcast (no good for photos) so I turned the Wave on High (High = 6000BTU) to heat up the chilly camper and went back to sleep for a bit. Some time later (45 minutes?) I was awakened by the camper's screeching CO detector. I turned off the heater and opened the camper door. "Hmmm...well, good", I thought, "the CO detector works and it's loud enough to wake me up!"
What I had done wrong, what caused the CO build-up (I think) is that I didn't open the vents wide enough for that High setting. I usually run it at Low or Med, only run it on High for the first 10 minutes or so to get the catalytic material heated up and functioning...but this time I went to sleep and didn't turn it down. So, not enough oxygen was coming in (from the slightly-open vents) to feed the heater for complete combustion so CO was produced, AND the CO that was produced was not being vented well. CO detectors are set to alarm at a level WAY below the dangerous level -- I felt fine.
I still use the Wave all the time in cold weather -- I love it; completely silent (unlike the noisy-fan forced-air furnace) and consumes no electricity. I vent the camper by opening a turnbuckle port at the bottom and the roof vent at the top. I use it on High, too, when it's really cold, but I open vents wider.
When it's really cold I do leave it on overnight, but only on Low -- and with enough ventilation.
And I haven't died yet from asphyxiation.
Your situation, with 4-6 air-breathers in the camper consuming oxygen in addition to the heater's consumption, is different from mine. For example, when I don't use the heater -- when it's cool but not cold -- I frequently close all vents/windows of the camper overnight, while sleeping...I bet you don't do that. So, you'll want to vent more than I do. The heater includes recommendations for how much vent-area is required, I believe. Use that as a minimum.