Do you have access to a multimeter that can read Amps?
It might just be an old battery that has reached end of life.
Or something on the van is drawing an excessive current whilst parked up.
A trustworthy battery fitting garage should be able to test the battery for you and advise if it is worn out.
If you have a multimeter you can check the amount of current flowing out of the battery by using the Amps setting on the multimeter (with the multimeter leads plugged into the appropriate sockets on the meter).
Switch off the engine, leave the bonnet open, make sure all the doors are closed and no interior or parking lights are switched on, radio etc.. Disconnect one of the battery leads and put the multimeter between the battery terminal and the battery lead. This should then give a reading on how much current is flowing out of the battery when parked up. After a few seconds or so, I would expect to see a current of around 40 to 50mA maximum. Anything much higher than this would indicate that something is drawing power from the battery and causing it to go flat. It's an over simplification, but if the draw was 0.5 Amp, then a 75Ah battery would be discharged in 150 hours.
Don't try turning on the ignition or starting the vehicle with the multimeter attached as above, or a much higher current will flow through it and could damage it.