Moose on the Loose.
The problem with van battery chargers, and set rate chargers, is that they are designed to charge the battery but not produce any gassing, as a safety precaution.
To get the most out of a leisure battery, it is best to use a multi stage charger which takes the battery up to 14.5v to 14.8v.
A freshly charged battery will lose some of its charge after the power is switched off and it has settled, and your reading of 13.7 down to 12.8 is about normal, and if left without charging, it will continue to lose charge over time due to the natural discharge rate of the battery and what it is made of.
To confuse even more is the power of the battry is measured in amps/hour, which is how long will a battery last drawing a set amount of amps over a set time, so a 110amp battery will, or should, last 110hours at one amp per hour drain.
If your battery continues to lose over a short period of time, like a week, then it is likely that there is an internal fault causing a problem.
Batteries in storage "should" be charged every 2 months to maintain their effective life.