Sounds like your batteries shot,
Once a battery goes dead it never comes back to full power again. And if your battery is between three and five years old that's about all they last anyway. Probably the easiest and cheapest thing you can do, and you don't depend on your batteries for doing very much, is get two actual deep cycle, golf cart style batteries, these are normally 6 volt so they will have to be hooked in series to produce 12 volts.
Another thing you can do is get a 12 volt AGM battery, this is the best way to go if you don't have room for two 6 volt golf cart batteries.
The brand of the batteries doesn't matter all that much because there's only a handful of companies that make batteries in America so most batteries are just rebranded from the same company.
You didn't mention how your battery is being charged, even the best battery won't work very well, or last very long if it's not being charged properly.
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