I "hard-plumbed" a Big Buddy Mr. Heater into our motor home. The Big Buddy has the low-pressure quick connect on it and has 9,000 - 18,000 BTU settings. Also has a fan that can be operated by battery or 110 plug-in. This heats up and keeps warm a 31.5 foot "leaks like a sieve" 1984 MH.
We use it when skiing at our favorite local ski hill. We are in and out quite a bit. We provide ample ventilation ... and no, this does not defeat the purpose of it, it keeps the MH warm and comfortable even on med or low, most days. If it gets really windy and cold (<10), we may have to run on high.
They are nice heaters. Very economical compared to the furnace, both in electricity consumption for the fan and propane consumption.
Would I use them to sleep??? Haven't yet. We always have electricity in the winter if we overnight and just use our furnace to bring up to temp and then our oil-filled electric heater to maintain overnight. If we didn't have electricity, I would just get it really warm at night and then use our -20 down sleeping bags.
But, during the day, absolutely. As JerryBo66 did, I also closed our MH up tight and ran the Mr. Heater Big Buddy full blast with no ventilation, other than the natural ventilation. I had our MH CO detector and our house CO detector and neither one went off after 8 hours. It was about 20 degrees out with a pretty good wind and inside was approaching 80 degrees, way too hot. But there was no sign of CO problems.
Now, there are many here that say never use, many that say they use them all the time, and some that say they use them but not at night. I guess I fall into the latter...