Same with my battery.
Took it to the autoelectrician and he measured everything and it looked good. Then he charged it up and still looked good. After a while it lost voltage and looked only fair. He said it was borderline. I decided to buy a new one.
Deep cycle batteries (he said) are designed to go flat and charge up repeatedly. He said if you were to leave it over winter you should charge it up on your home charger every couple of months otherwise it sulpherises or oxidises (or some such word) and will never quite be the same again.
As for the borderline battery, he said it wouldn't be the right battery to start the Commodore but would be OK for the Toyota (except it doesn't fit, LOL). Deep cycle doesn't have the big amps to start a big engine, rather a steady amp drain like all the eqipment on the boat, tilt, starting outboards etc.
Give a man a fish and he will eat for a day.............Teach a man to fish and he will sit in a boat all day drinking beer,