Echoing others...
The short lifespan on your marine battery suggests a systems problem or abuse.
If you have a systems problem, you'll ruin new batteries just as quickly as the old. Get the right tools to check it and verify that it's not ruining your batteries. Others touched on this well, so I won't repeat.
My hunch? Let's assume "abuse", because that's most likely.
Abuse comes generally in two forms.
1) Most common is discharging too deeply. The typical deep cycle battery can deliver about half of its rated amp hours before needing to be recharged. So a group 24, the most basic that comes with most RVs, is rated around 75 amp hours at a certain rate of discharge (see below). You get to use only 35 to 40 amp hours without harming the battery. Bigger batteries hold more, but the rules are the same...you get half. The battery keeps half.
2) Poor maintenance - failure to keep it charged (sitting around dead is bad); failure to keep it full of distilled water. A heavily exercised battery goes through LOTS of charge and discharge cycles during a camping season. All that activity can (I didn't say will, I said CAN) "boil" off the electrolyte (boil is a deliberate exaggeration). And evaporation is a factor in vented batteries 24/7. I top off my battery bank about 4 times a summer...at minimum. As you know, if the plates on the battery are exposed to any degree for any significant amount of time, you run the risk of ruining the battery.
So there it is. If you manage the battery properly, even a Walmart marine battery will give good service for 5 years or more. Your experience suggests "abuse."
Now, a fiver with a single 12 volt battery? WHAT? That is a recipe for disaster. Ludicrous! A fiver needs lots of power. Even a single 12 volt golf cart battery will be taxed to its limits trying to keep up with the draws from fiver-sized loads. Furnace, water pump, ignition for the fridge and hot water heater, lights, entertainment (?), exhaust fans (at the stove, bathroom, fantastic fan), parasitic draws, and who knows what else. But the furnace alone is a real load--even in a small fiver - 5 to 10 amps or more when running. You're stuffing 10 pounds of loads into a 5 pound battery!
So, assuming your rig isn't ruining your battery, you need to do some math. Get out your manuals and determine your draws. Use this calculator tool.
https://www.rapidtables.com/calc/ele...alculator.html
Also understand that voltage is ALWAYS 12 volts even if you are running a 120 volt appliance through an inverter, because when you're boondocking on battery, the ultimate power source is
always 12 volts.
Quick illustration. I use a small inverter to run a 120 volt, 360 watt electric blanket for 15 to 20 minutes to take the chill off the bed. This is a HUGE load on the battery. Plug in 12 volts and 360 watts into the calculator and you get 30 amps!! Run that blanket for just over an hour and the entire usable capacity of a group 24 battery is gone! 15 minutes at that rate = about 8 amp hours (allowing for inefficiencies in the inverter, etc.
Do your math and recognize that your poor, solo little battery is outmatched by the rig. Even a Trojan AGM Group 31 12 volt is only rated at 100 amp hours at the 20 hour rate (the normal rate used for this calculation), and that means you get to use about 50 of those amp hours. Period, end of story.
https://www.trojanbattery.com/pdf/da...ata_Sheets.pdf
As you know, I'm "accusing you of abuse of battery"...a common practice and certainly likely when a fiver is sucking off a single 12 volt teat. This is a very common mistake, and I'm not really accusing.
The good news is that "Golf Cart" batteries CAN be (not necessarily are) more tolerant of deeper discharges. Some as much as 80% instead of 50%. LiFePo batteries (think megabucks) most definitely are more tolerant, but they typically require upgrades to your charging system, because they tend not to like the garden variety junk supplied for flooded cell batteries.
https://www.google.com/search?q=life...hrome&ie=UTF-8
Again, assuming your fiver's charging system isn't the culprit, the "right answer" is to find a way to cram 2 x 6 volt golf cart batteries somewhere in the rig. Wire's cheap, and you could, for example, put the batteries into the storage compartment up under the overhang, between the landing legs. You'd need to vent the space, but it might be worth the effort. 2 x 6 volt golf cart batteries combined will have a rated capacity around 230 AH, and you get to use about 115 of those.
Next, spend $600 and put 400 watts of solar on the roof so you don't need to listen to the generator for hours on end.
https://www.amazon.com/Renogy-Monocr...0411342&sr=8-2
Windy Nation makes good stuff, too.
Next, if your rig has incandescent bulbs, change them for LEDs on all interior and exterior "camping" lights (don't worry about the road lights).
Next, if you're serious about boondocking, install a "kill switch" into the power feeds to the stereo/DVD player, TV, and so on. If you MUST be entertained, get a bluetooth speaker, or turn on the stereo briefly while listening. But these devices add significant parasitic loads that are on 24/7...so much so that homeowners are encourage to disconnect these things with power strips when not in use.
Finally, don't run ANYTHING requiring 120 volts through an inverter unless you want to watch your battery's capacity evaporate in minutes (see electric blanket example above.)
Here's the fact. Batteries SUCK as energy storage devices. The facts of life if you will.
~ The gold standard - 4 x 6 volt golf cart batteries - has a usable capacity of 230 amp hours.
~ 230 amp hours equals an energy equivalent of 2.76 kilowatt hours:
https://www.inchcalculator.com/ah-to-kwh-calculator/
~ Propane contains 91,000 BTU per gallon. So a 5 gallon tank (about 4.5 actual gallons) contains 409,500 BTU.
https://www.google.com/search?q=prop...hrome&ie=UTF-8
~ Your 5 gallon propane tank contains the energy equivalent of 120 KWH.
https://www.google.com/search?q=conv...hrome&ie=UTF-8
~ One puny 5 gallon tank of propane contains 43 times the energy of the gold standard battery bank.
~ And that's true of a $2000 pair of LiFePos, too.
BATTERIES SUCK as energy storage devices. If you're into boondocking, you're well advised to know your loads and manage your batteries (plural intentional) carefully.
One last tip. If you're boondocking, SETUP AND RUN THE GENERATOR to power the landing legs, slides, and other large 12 volt loads when setting up. Leave the genny running as you finish setup. Listen to the "converter" inside. If the fan is running, it's charging at a high rate. Don't turn off the genny at least until the fan shuts off on the converter...a sign that the charge rate has stepped down. If you have that nice solar kit on the roof, it will take over from there. BUT, 400 WATTS OF SOLAR WILL ONLY PROVIDE ABOUT 120 AMP HOURS OF CHARGE PER DAY under ideal conditions (e.g. Sunny Colorado). Clouds, shade, spring and fall days with lower sun angle, and so on will reduce that output. And every amp hour counts, whether consumed during the day or night...the panels put out 120 amp hours, that's it, that's all. So power the jacks and slides with your genny.
End of rant. Oh yeah, who in hell designed a fiver with room for only one battery?? It's time to call in the firing squad.
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