Canvas pop up?
I would pick the 6,000 Btu model.
I have a 6,000 Btu 1992 model Olympic catalytic heater in my motorhome that heats a well insulated 30' Bounder to around 70F when it is 40 or warmer outside. Below 40, I still need to run the furnace a bit to keep it at 65-68 inside. Yes I like it warm.
And you need to keep a roof vent partway open (about 3/8" on all three vents in my case) and I keep the window near the kitchen sink open about 3/8" too.
Several times I figured that opening all those windows was not worth it, and run the furnace when it is in the 55 - 60F range outside. Running the heater, even on low output would have overheated the living room where it is located.
Yes they are safe to use inside, my CO meter never tripped.
Fred.