Electric heater that use the 1500 watts to heat an element will produce about 5200 btu's of heat, MAX. Obviously on a medium or low setting, the heat output will be less.
If you want to spend more money for the best, you can more than double the btu output of any electric resistance heater with a portable heat pump that operates off of 120 volts. These can put out 10,000-12,000 btu's of heat and still use less than 1500 watts.
I use a portable heat pump/AC unit when we travel and have no problems even in the coldest near zero temps. Mine vents to the outside through a single hose and it produces 12,000 btu's of heat using less than 11 amps or 1300 watts of power. It also has the advantage of removing moisture from the coach, which is great in the Winter months. These are not the cheapest but if you travel a lot in very cold temps or very warm temps, they can help keep you comfortable. Since there is NO heating element, they are also safer around pets and people.
Portable heat pump