Agreed, remove the one and carry on with the other untill BOTH can be replaced. A hot and smelly battery is telling you it could be becoming dangerous.
One old and one new battery is not a good idea, especially if the older unit has been worked hard. You will monkey the new one. If the age were only a few months of easy use it might be ok but not from what you have described.
It sounds like you may have a heavier load than you realize or you are waiting too long for recharges. Many people underestimate the loading. Even a good fridge with the Danfoss compressor, a 3.5A, can use a lot of power in a day, especially if hot. The duty cycle can be a lot higher than realized meaning more A.H. than realized. Add an inverter for a microwave or hair drier and that load really jumps.
Yes, you can run them down to below 50% before a recharge but at a steeper cost of life cycles. Take a look at the mfgrs. website. They usually show a cycle life vs discharge depth.
How much time do you recharge? AGM need to be driven up to full charge often for good life.
Charger? Is it working properly and is it big enough to recharge the batteries in the time available. Same for the alternator.
Two 4D AGM? Fridge, water pumps, lights, hot weather. To me that is two days. I have three 8D gels and get three days with one fridge, led lgts, water pump, and charger for cell and tablets.
My charger is not great but I have a 150A alternator controlled by an Ample pwr. regulator and a good run [1-2 hr] will get me another two days.
I bought a chest freezer which we use as an extra fridge and it added enough load to cause trouble. I knew better but where we go ice is no longer easy to find if at all, thus the extra unit.
When running daily there is no trouble but of course once at our favoured cruising ground that all changes and every once in a while I must head for the dock for a real overnight recharge.
Take a good hard look at battery capacity, loads, discharge depth vs expected life before recharge. If you are in a sunny clime then maybe a couple solar panels would be a good investment. Even with out enough panels to carry the full load they may give another day or two without damaging the batteries with too deep a discharge.
Lots of google info, magazine articles on working out amp hour use per day. That needs to be done or you may repeat especially if the battery capacity is insufficient.
