The electric option, you pay for, in fuel and alternator wear. 300 watts is a small heater, 1/5 of a typical "milkhouse" space heater. But that's 25 amps at 12 volts, a pretty heavy load.
Still, as an electrical worker, I must admit it would be very easy and simple to install. It will be a bit more work to plumb up for a "rear heater" in the trailer. Not rocket science, but work.
Hey, I have an unused 1996 Suburban with dual heat and A/C. Just take that unit and run some longer lines, right?

Life is too short not to be happy.