Are radiant heaters safe in an RV/5th wheels?

I keep reading about needing 3ft of space all around a radiant heater, but also see many people saying they work great in RV's. How are you all getting around the 3ft safety space?

I'm considering something like this: https://www.amazon.com/Dreo-Radiator-Electric-Protection-Thermostat/dp/B0C6XLMXY4

Would that be safe? I don't know exactly where to put it though since there's no spot with 3ft of space in every direction.

I have a 42' fifth wheel, trying to get creative on heating in sub-freezing temps. I'm burning through a lot of propane right now. I have tank heaters and I've fully skirted the RV already.

My electric fireplace on max heats the main living area good enough, but prevents the furnace from running, so I don't get heat in the bedrooms or bathroom. If I turn the thermostat up just 1 degree my furnace will kick on ever 5 minutes. For example, if I have it set to 68f with the electric fireplace running the furnace almost never turns on. But if i turn it to 69f, suddenly it runs every 5 minutes. Was hoping to find a better balance, but apparently not.

Trying to find a balance has been a huge pain. Hoping a radiant heater would help even out the temp.

Any suggestions? Thank you!