I think you'll find the reviews from people who have owned 12 volt heaters are generally not great. Also, at 600 watts (50 amps DC), a 100 amp hour lead acid battery will provide an hour of heat, maximum, under perfect circumstances with a fully charged battery. A 100 amp hour LifePo4 battery will provide a little more than an hour and a half of heat at 600 watts. Not meaning to state the obvious, but then you have to recharge it to do it again tomorrow.
The other option involving no installation is a portable propane heater. These come with risks you should investigate thoroughly. These risks include the possibility of depleting the oxygen in an enclosed cabin as well as filling a non-vented cabin with toxic combustion gases.
I've tried the Mr Heater Buddy. In my opinion, it's not a good solution because (1) It puts out 4,000-9000 BTUs of heat which is too much under most circumstances for a 26 ft boat especially in South Florida even on the coldest days, (2) in my personal experience with the Mr Heater Buddy the low oxygen alarm is so very quiet that it is not sufficient to provide a warning, (3) they burn a lot of propane and create a lot of condensation, (4) though people sometimes mistakenly refer to them as "catalytic heaters" there is actually nothing catalytic about them, i.e., they do not catalyze the propane into heat, instead they burn the propane in a fire.
After much (much!) experimentation with heaters on boats and campers, in my opinion, the best solution for occasional heat that does not require an installation is a Camco Olympian Wave 3 heater.
https://www.google.com/search?q=Olympian+Wave+3+heater
It is actually catalytic, i.e., there is no flame except for the initial pilot light which goes out once the catalyzing process begins. Otherwise, the heat is produced by catalyzing the propane in conjunction with platinum. It provides 1600-3000 BTU, giving you the ability to leave it set on low for extended periods that will raise the temperature of the cabin 15-20F, great for those days when it's chilly but not snowing. It will raise the temperature in the cabin 30 - 35F running on its highest setting.
One pound of propane provides 21,500 BTUs of heat. Thus 1 lb bottle of propane will provide somewhere between 7 - 13 hours of heat with the Olympian wave, depending on how you have it adjusted. Compare this to the Mr Heater Buddy which will provide 2.5 - 5 hours of heat for the same amount of propane.
I've attached a photo of the stand I built for my Wave 3. This included
- small piece of plywood
- short piece of 4-in diameter PVC tube
- black spray paint
- adhesive (5200) to attach the PVC to the plywood
- screws to attach the heater to the plywood.
- propane adapter to use the heater with 1 lb bottles
https://tinyurl.com/y6j4r88n
Also I drilled a small hole in the stand and another in the table in my 26x to attach them with a bolt so I can rotate the heater in any direction I like, but the heater will not bounce off the table even in the rough conditions.
The Wave 3 seems to create very substantially less condensation than the Mr heater Buddy, but that could be primarily due to the fact that it consumes so much less propane.
Finally, note that a variety of vendors now sell legally refillable 1 lb propane bottles. I have a number of these and just refill them as needed. In my area, this ends up costing approximately 85 cents per pound for propane.
Finally, if for some reason you find the Mr Heater brand more appealing, they also sell a line of catalytic heaters which are essentially copies of the Wave heaters, but they are more expensive than the Wave.
https://www.mrheater.com/product/heaters/catalytic.html
[In the interest of full disclosure: I primarily use this heater as a supplemental heater in addition to the diesel heater in the boat on those occasions when it is very cold, below 35F. I have recently installed a vented propane heater to use in addition to the diesel heater for those situations. Nonetheless, I found the Wave 3 to be a great solution for occasional use.]