Leisure battery

From what you say the battery is old and tired. If it is the original it is past its normal working life. 

Disconnect the battery and measure the voltage after you have charged it, it should be 12.5 to 13v ish.

Connect it up and use some power, switch the habitation lights fan or anything else for 15 min that has a load. Disconnect and measure the battery. Disconnect and measure the battery again, if it has dropped below 12v then its finished.

Alternatively borrow a battery tester this will confirm. 

If you bought it from a dealer then I would certainly complain.

Also, at 3k miles a year from new the vehicle battery needs a load test as well as it has had a hard life.

Leisure batteries need to keep at least 50 percent charge .I suggest its been left and not maintained . A lot of people put van away for winter , come spring cant understand why battery wont hold charge

I would just buy a new battery because it sounds as if its too far gone. My choice would be a sealed marine battery. My one year old motorhome has a Banner battery but it seems to require regular topping up which can be forgotten.

David

Thanks for replies. I'll get it checked at my local motorhome garage, get them to say its  no use and then contact the dealer. It was checked ok on the pre sales list so they should be liable for a replacement. 

I have a CTEK battery charger which has a recondition mode which will recover deeply discharged batteries. I have used it many times on old batteries, and it will restore them, not quite as good as brand new, but certainly to make them usable again. The model I have is very old and no longer current. The equivalent model now is MXS 7.0 UK. See https://www.ctek.com/uk/products/car/mxs-7-0-uk 

However, these are expensive. You sometimes see them much cheaper, but they will be copies, so beware of any deal that looks too good to be true.

CTEK are not the only company making advanced battery chargers. If you search for "battery reconditioner desulfator" you will find plenty of other options.