Hi,
My belief is that the oil filled heaters are the safest you can use. I've worn out two and may be upgrading to baseboard heaters this winter.
There are some 1/2 size oil filled and some flat panel ones that take no extra space at all. I do keep oven liner tinfoil to protect the walls of my RV and reflect the heat into the RV.
If you plan on using the full 1500 watts make sure to replace the receptacle in the RV with one that can take that sort of load. The OEM stab type are not suitable.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.