After all that, there are two types I would personally use.
My top pick would be a built in toe kick heater- they are hard wired, and can draw 1800 watts, so put out more heat than a portable. You do need to have a place to install it, and they are in the $200 range.
I don't know if anyone else makes a heater that works like this, but the original Pelonis is my top pick for a portable in an RV- they work unlike others, in that the heating element is a PTCR* element, and is inherently self limiting temperature wise- the element simply cannot get hot enough to ignite anything. The thermostat then varies the fan speed only, but because of the PTCR, more air flow over the element pulls more heat out, cooling the element and drawing more power. Others may work like this, but I haven't found one, even though they say "ceramic", they are more conventional.
*Positive Temperature Coeffecient Resistor