We have the Big Buddy heater and have used it to heat our RV when we're not connected to power. We have a propane hose that's connected to the RV's propane supply and it connects to the low pressure quick connect on the Big Buddy heater.
We run the heater all night on low with a vent and side window cracked. Have done this many times and we're still here.
They are as safe as any other propane appliance as long as they are in good condition and you follow the safety guidelines.
We don't run the generator in the RV, and we don't run the BBQ in the RV, but we read in the paper every winter where people die from doing just that.
Some people can make anything dangerous.