Voltage with batteries at rest can be a guide to the SOC (State Of Charge) ie half full, half empty etc.
but
is not a guide to the capacity (amount of energy stored).
You can get a 'high' but there is not much left in the tank.
Example:
my battery bank was 330 amp,hr when new but with age and use they now only have a much less capacity possibly about 80 amp.hrs, when I charge them the voltage will rise and the Smartgauge will say they are at 100% SOC after a few hours.
I then use the batteries but they will only supply energy for a much shorter time than when they were new.
So to summarise the batteries are full but only to the capacity they have.
Used this before to give an idea:
think of the battery like a bucket, you fill it and empty regularly with time the bottom fills with silt/gunge etc. you can still 'fill' it but it does not have as much capacity.
If you regularly clean the bucket or fully charge the battery it will maintain a higher capacity for longer.
ps. A start battery is not the best, to use for running the domestic side of a boat.
You can wreck a new battery in days if it is not charged correctly, so do not buy a new one unless you plan to monitor and charge it correctly.