Webasto has a self-install kit for diesel furnaces.

I put one on my boat last fall, and it didn't prove to be difficult at
all.

You would want one of the smaller units, and you would have to install
a tank for the diesel. The diesel option is likely to be more effective
and safer than propane or alcohol.

Another good option might be a Wallas stove. They are commonly found on
boats of your size category.
Do a search for Wallas on the internet, and you'll surely get a bunch
of hits. (You'll still need a diesel tank).

Send me a good email address if you'd like the text of the
heater-installation series.