12 volt compressor fridges are a VERY poor choice for boondocking. The "old fashioned" 2-way fridge that runs on 120 volts or propane is the right choice for "dry camping."
[END OF RANT]
By all means, investigate the power consumption of your fridge. It sounds as if it's abnormally high.
Do the math, and you'll see that you should expect, in round numbers, about 3 amps load for maintaining, and let's assume a 50% duty cycle. That equals 3 x 12 = 36 amp hours for battery consumption per day.
In your case, you have what I presume are two group 24 marine batteries rated at a generous 75 AH capacity. You get to use half of the total 150 AH (75 AH)...which means that you SHOULD be able to last 24+ hours with the fridge, furnace, water pump, spark ignition for the water heater, lights, awnining in and out, and, if you have one...slide operation.
That's the picture. On a good day, you get one day...but not 5 or 6 hours. Figure out why your fridge is so power hungry...or your batteries are shot.
Next? You need, in order of importance:
- More battery...LOTS more
- Solar...and lots of it.
I have a 2 way fridge. I have two 6 volt golf cart batteries
(GC-2s) in series with a total USABLE capacity of about 115 AH, and I have 400 watts of solar on the roof. We don't worry about 12 volt power. I also have a generator, but I only use it for 120 volt loads...like the microwave and espresso machine. After all, we aren't savages.
My points are:
- Check your batteries. Maybe they're toast (see P.S. below)
- Investigate why your particular fridge is such a power hog. All 12 volt compressor fridges use lots of power, but yours is off the charts.
- You need LOTS more 12 volt storage (battery) and generation (solar and generator) to make this thing work when boondocking.
12 volt compressor fridges are horrible choices for boondocking. They are wonderful fridges for RV parks with hookups. Manufacturers are moving to 12 volt compressor fridges, because they are cheaper to install, have 40% more capacity for their given physical size, and anyone can use one....unlike propane fridges which require a modicum of skills.
Re-read the rant.
You own a 12 volt fridge, so make the necessary adjustments so you can enjoy your RV in the boonies.
P.S. If you have repeatedly drained your batteries "dead", they are ruined. You can only use half the capacity of a FCLA (flooded cell lead acid) battery. If you drain it dead...especially several times...it's ruined and won't hold a charge.
So, before you go bananas investigating the fridge, think long and hard about the possibility of past battery abuse. Maybe your fridge is fine, but the batteries have been drained dead over and over, so they won't hold much charge...thus the 5 to 6 hours of life with the fridge.
This also presumes that the electrolyte levels have been maintained, AND that one of the batteries hasn't just failed...as things sometimes do. Take your batteries to an auto parts store and have them load tested...and you'll know.
P.P.S. Batteries good and fridge performing as it should? Now you're stuck solving a bigger problem. What's eating your battery alive that you don't know is doing it? A mystery power draw can't be ruled out if none of these suggestions pan out.