My house batteries are Group 24 12-volt deep cycle from West Marine. I got them in the spring of 2017. They are serviced each fall and put on a trickle charger. As soon as I turn off the battery charger at the dock, or shut the engine down, the indicator on my instrument panel shows a pretty quick drop from 13.1 to about 12.4 in maybe 15 minutes.
13.1V is not an actual "charged" voltage for a flooded battery, it is a
surface charge. The full resting voltage for that battery will be around 12.70V - 12.73V. When you place the battery under load voltage will fall even lower.
Please also understand that not all "trickle chargers" are a quality product. Some battery makers such as Lifeline specifically prohibit "trickle" (constant current) charging. Many of these "trickle chargers" are nothing more than a low constant-current and if left unattended can literally cook your batteries pushing them well past 15V.
The best maintenance chargers will be a constant voltage design that drop to a
float level voltage 13.2V to 13.6V after charging to an
absorption level 14.2V - 14.8V.. Like most things you do tend to get what you pay for in battery chargers.
Where you're measuring the voltage, and what device you use, can display even lower than the actual battery voltage, if wiring is not in perfect condition and sized for the load or the meter is inaccurate.. The only measurement that matters is directly across the battery terminals using a known accurate DVM or volt meter.
Bottom line? Measure the actual battery voltage under load and don't trust your "indicator" on the instrument panel. These are quite often rather inaccurate. We routinely see the analog ones displaying more than 1V off actual, and even the digital ones showing as much as .6V - 1V off actual battery voltage.
Also, your batteries are really more of a dual-purpose than an actual deep-cycle product despite what the misleading little sticker claims. The article below goes into detail on this subject..
What is a Deep Cycle Battery? (LINK)
