Accessory mode or Inverter for 12V cooler?

Your batteries are bad if the inverter is going into low voltage alarm with just 75 watts of energy being used.

 

3 batteries or 4? need 4.

 

I can tell you that those coleman units use more than a guy would think.
I know quite a few that switched from them to an inverter and small 110 volt fridge with much better results, I solved my problems with just chunking my coleman and not keeping a fridge in the truck. I prefer to stop and have a meal anyway, so that was an easy decision, except for when I ran the alcan hot and heavy, finding good places to eat was somewhat of a challenge.

 

Coleman's are fine when new. But the older it gets the more power it consumes.

 

need to keep the heat sink fins cleaned off.

 

You lose a lot of efficiency converting the power twice. 12 volt appliances should be run on 12 volt directly.
On my Cascadia the 12 volt outlet to the right of the cup holder is on at all times.

 

You are converting the power twice (12vDC--120v AC--12v DC) which uses/wastes more energy before it gets to the cooler.

Run a longer cord to an outlet that works or wire one of the rear ones to be on all the time. You can get a cord with a lighted switch on the plug.

I still have a Coleman from 2000, replaced the power slurping fan motor with computer fans inside and out and it runs with the plug a lot cooler.

 

I know, and I do.
But the older it gets the less efficient the cooling element (whatever it's really called) becomes.

 

You can get a new Peltier chip online but mine still works fine. The original electric fan motor was design failure, sitting at the top of a clsed space above the hot side heat sink.