Mr. Heater Big Buddy Indoor/Outdoor Propane Heater

Look into the Camco Olympian wave heaters, safer units from what I have read and use less propane too.
3000 btu unit is listed as suitable for 130 sqft, I know canvas sides lose a lot, but a 6000 may well be enough.

I dont know offhand the specs of the furnace in our camper, but our pup is a little smaller. Nonetheless, there is a carbon monoxide detector on the inside. Yours should have one too. You can locate it in the manual (or online) and test it similar to a standard smoke detector. You could buy an after market one as well, but I'll bet my dollar that you already have one.

I have one.. puts out a whopping amount of heat with no CO worries.

They create a lot of moisture unless your someplace really dry. Also, while they are pretty safe I would not sleep in a small area with one running.

On high mine will heat my big 5er from 60 to 70 in about 5 minutes. But I still prefer the furnace. Quieter and more even heat. And Mr Heaters really go through those cylinders fast on high.

I'm in a dry climate so I didn't notice any moisture.

We used a pair of buddy heaters to heat our travel trailer while dry camping in a state park. They worked great, but I did find myself double checking the CO detectors.

We use a ceramic (electric) heater in our TT and I usually have to turn the thermostat down as it gets to darn hot.

I have one of the smaller single disposable tank heater buddies which has served me well over the years. It is the older design with the non-swiveling tank mount. Though these are relatively safe, I would not sleep in a confined space with one running. One issue that does come up on mine when running in a cooler environment is freeze up on the small disposable 1 pound tanks, if ran on high in a cold (near freezing temperature) space the tanks will tend to freeze up and the heater stop working when they are still 1/4 - 1/3 full, I don't know if this is an issue with the dual disposable tank models. Tanks left connected will also tend to drain down over long periods of time (weeks or months)

There are three heaters in the Buddy line. Little Buddy, Buddy, and Big Buddy. The Big Buddy used to have a quick connect port for a hose that could be connected to the low pressure side of the propane feed. This would allow you to plumb in a hose inside the RV from the cook-top, furnace, water heater, or fridge. I don't see that mentioned for their current model.

I have the Buddy that I use in my 24ft motorhome and it works well. I removed the built-in regulator and attached a hose that is plumbed into the line that feeds my cook-top.

I would think that the Buddy would be large enough for your needs. If you do not want to remove the built in regulator there are hoses available to connect direct to a propane tank.

I have one, but I've never used it in my pop up. I usually camp at a spot with an electric hookup and just use a ceramic heater which works nicely to heat the popup. I wouldn't hesitate to use the Mr. Heater Buddy heater in my popup when awake. Like others have said, I'd be hesitant to use it when sleeping. Anything with an open flame that puts out carbon monoxide (minuscule amounts but still) and burns oxygen is probably best used when you are fully awake.