When in the marina...
we have a heat pump to rely on for warmth, as do other big boats in our marina. We also keep a small cube quartz heater as a supplement. Others use one of those oil-filled radiators bolted down to a flat board to prevent tipping. Only a few have something like Ross's Force 10 heater, much more common further north and out in the Northwest.
If you use any form of combustion for heat such as the Coleman unit, you MUST install a CO detector for your own safety! The danger lies in the CO from combustion, not the propane in a well made product such as Coleman's.
I must admit that SailM8 has the best answer!