Bongo Fury

I've had my Bongo for about 4 months, it's unconverted but had an EHU and Leisure battery fitted by the previous owner at Southern Bongos in November 2022.

I bought a 12v usb adapter for the socket to connect to my leisure battery to use temporarily with the engine off. I connected my battery tester to the battery (12.70) and plugged in my son's tablet to see if it actually worked. It was charging but was dropping 00.01 roughly every 6 seconds or so. On this basis it would be dropping 01.00 every 10 minutes, meaning a small tablet would drain the battery quite quickly. Is this correct or have I got it completely wrong?! Essentially I want to know if I can charge things or use a 12v cool box with the engine off while camping with no EHU.

I've tried googling it and most answers say that a leisure battery should run a fridge for a few hours. It's a Probat battery which I'm reliably informed isnt great, but surely should last longer than 2.5 years? Particularly when the installation cost £300!

Thanks in advance.

Right several questions and some background;
All Lead Acid leisure batteries will have a "resting" voltage (i.e. no current being drawn or charging current and allowed 30-60 minutes from last charging) of around 12.6 to 12.8 Volts, the precise voltage will depend on temperature and various additives used in battery manufacture.
The voltage alone does not really show the state of charge of a battery, merely that the normal battery chemistry is taking place, you will find that the voltage will rise quite a bit higher when the temperature is lower and lower when the temperature is higher!
If you really want to know and understand the amount of charge in your battery then you need to fit a Coulomb Meter (Coulombs are the measure of charge unlike Voltage and Current which depend on the load running from a battery) these are quite cheap these days something like this one https://www.ebay.co.uk/itm/396209169673
My own just sits quietly in the glove box and gives a continuous display of the Capacity left in the battery, Voltage, Current, and time to fully charged; Now to further questions, do you know the capacity of your leisure battery probably marked on the battery 60-120 AmpHours are common values ?
Do you know if you have a Split Charge Relay ? This charges the battery when driving, it splits the current from the alternator so that both the starter and leisure battery get charged together when driving, but then isolates the starter battery from the loads when then engine stops.