Your problem isn't the size of the battery, but how you use it. It's likely never fully charging, and it's being constantly run down by the fridge, so it sulfates and slowly dies. ANY battery will have the same issue. Remember the last few amp hours of charge takes ages to finally get into the battery and a 20 minute drive to work may get it to 90%, but not to full charge. That last 10% is the problem and takes hours, not minutes.
The solution is to plug the car into a batteryMinder or similar at night. It will fully charge and then float the battery-this will keep the battery healthy. It would also be better to leave the fridge off which lets the battery stay at a higher state of charge.
And it would be better practice, if you need to run the fridge, to have a second isolated deep cycle battery to run the fridge, and leave the starting battery out of the picture.
All batteries do best if kept at 100% state of charge all the time. Anything less, starts the sulfation process and leads inevitably to the premature death of your battery.
Me personally, I like the BatteryMinder charger. It's a true temperature compensated, stepped charger, isn't expensive, and keeps the battery at 100% state of charge all the time. Or you can replace your battery every year or two.
I think this is a good plan. The Grp 31s do not fit well. Wet cell starting batteries are fine, just remember to check the electrolyte level from time to time and top with distilled water if needed. And keep it on a battery maintainer. This will easily double the life of your batteries. A battery maintainer is not the same thing as a trickle charger. So spend the $75 on a BatteryMinder and forget about battery problems for the foreseeable future. If you don't want to plug it in at night, then don't run your fridge so the battery stays at a higher state of charge.