I bought this little 400/800w heater to use in my 13' Scamp
(on electric sites of course) as the nights got chilly camping this fall
(into low 20's F).
Here's a link to Northern Tool
for more info.
What I like about it is it's very lightweight, can be stored in the water tank compartment, and it's very quiet so I can hear the birdies chirping in the morning at my campsite.
You do need to keep things away from it. I installed a 110 outlet below the closet and that location seems to work out well. Nice and toasty warm when I put my feet down on the floor in the morning.
It's on sale at Northern Tool + Equipment for $19.99; and if you got their coupon flyer in the mail, there's a $5 off purchase of $15.