An RV fridge will draw about 2-4 amp per hour depending on the fridge.
2 standard Rv/marine batteries say they have 75 amps when new, thatís 150 amps, but you should not use more than 50%.
I just did a draw down test on one I had that is 4 years old. I could only get 26amps out of it before it dropped to 11.7.
So, that would estimate you get 52 amps from your two, or course depending on the condition of your batteries.
Estimate is you will get 12-24 hours as long as nothing else draws power, like lights, CO2 detector, usb chargers and of course anything big like jacks, slides, heater, etc.
2017 Eagle 291RSTS
2017 Chevy Silverado Duramax 2500