I tested an oil filled heater in my 4x8 Benroy.
Did it work? Sorta...
Because there was no fan moving air around, I found that the teardrop got really hot near the ceiling, and was quite cold at the mattress level.
And it did take up a lot of room.
However, it was very safe!!!
To use something like that you almost have to design your teardrop around the heater, and add a fan to keep the heat from stratifying.
Mike...
FWIW, my next build will have a forced air furnace in it...

The quality is remembered long after the price is forgotten, so build your teardrop with the best materials...