Above are all options. Here are a few others....
When ever I get a propane tank filled at an RV store, one I have used for years, I scan their store shelves and catelogue for RV accessores...you'd be amazed what's there...like battery boxes for 8D's 40% below "marine discount". Or nice stainless steel coffe pots...Give that a try.
Also, you will NOT want a 1,000watt crocpot to keep a small amount of liquid warm. One option is to use a small coffemaker and keep soup in the coffee pot on the warmer...if you have an inverter a 120v type or a find a 12v coffee maker at an RV store.
When making long/early morning runs, I often get underway EARLY, then when in open water turn on my under shelf coffee maker...via inverter..in five minutes hot coffee via alternator power so I have NOT had to start a genny and waste time before getting underway. The pot sits in a recess in the maker so it never moves..
In any case, if you are underway when wanting to keep soup warm, alternator output is likely more than sufficient to feed a 300 or 400 watt heating appliance. My 1500 watt microwave pulls 144 amps at 12v via an inveter, but the coffee maker draws under 20 amps...If necessary, buy a small inverter...it can be used for household TV's, coffee makers, tools,etc,etc,etc, just not all at once.
This is an ideal example of when to NOT,NOT,NOT use a conventional diesel/gas marine genny!!!. It's extremely expensive to run a such valuable piece of equipment ($5K,$10K,$15K???) for such a tiny load. Lose water cooling just once and BANG!!! you may have big problems...On the other hand, if you are running the genny for other purposes keeping some soup warm at the same time is a no brainer....
Also, depending on your needs, consider a small portable gas genny. I carry a larger 4KW (Home Cheapo) in my cockpit and run that when just charging batteries in hot weather. (Only when other boats are not near as it's not quiet.) Hence I don't have a 98 degree engine room all summer long unless I choose to run my water cooled diesel genny.
