Yea the Big buddies are ok to use indoors. That being said, i always have both front windows open about 1-1.5 inches and i have a Co2 detector. It has never gone off and there has no issues at this point. The heater also as a "low oxygen" detector built in and shuts off if there is a loss of oxygen, that too has never happened.
The windows bein down a bit isn't an issue as long as i park the van so that thw ind isn't blowing right into it, and to be honest, i put the sleeping bag up right around my neck and i am good and warm, and i like breathing what cold air gets to me!!
All in all, i hve no thought of getting any other kind of heat as long as i am in a van.
PS: I have the heater set up to use a 20lb tank that i chain to the front wheel. It consist of a hose and filter, don't use a big tank without the filter, something about the oil in the fuel gumming up the works. I run it on low for the most part, have had it on med once and it was too hot. I haven't had any issues with the unit shutting off or with the frost issues that some have had with the 1 lb canisters. It all has worked great, and as a matter of fact, it is running right now as i type this
One thing, they make 2 models. 1 with just the capacity of 1 cansiter, 1 with 2. The bigger model also has a fan at the top. The fan doe NOT blow hot air. What it does is move the hot air off the ceiling and moves it around to heat the other areas of the van.
It has worked great for me.