Leisure Battery

We have a Nuevo 2, which we've had for about 3-4 years now.  Initially the leisure battery was very good, and with the solar panel recharging it each day, it gave us plenty of use each evening.  One evening, we accidentally left the outside light on, and ran the battery right down.   Since then, it has never been quite as good.     Recently, we have been finding that even after it appears to be fully charged, within a few mins of putting lights or TV on, the panel is reading "11.5V fair" for the leisure battery.   We tried charging it from a battery charger, but the same result.   So we decided to get a new leisure battery.   We charged it up fully from the battery charger before putting it in the van.   When we disconnected the old leisure battery, we were surprised to see the panel still reading 11.8V, but continued and put the new leisure battery in.    Once again, within a few mins of usage, the panel was reading "Fair" rather than "Good".    When we put a multi-meter on the old battery which we had taken out, it appeared to be at a higher voltage than the panel had been showing (I know this might just be tolerance on the readings).   We haven't been away in the van with the new battery yet, to know whether it actually lasts longer than the old one of an evening (more important than what the panel reads), but it seems like something is dragging the battery down when it is in the van.   As far as we are aware, the van only contains one leisure battery, but we are starting to wonder if there is a second one in parallel, which is older/weaker and dragging it down.  We've had a search, and not found any additional leisure battery (actually, we couldn't even see the driving battery when looking under the bonnet, but it must be hidden underneath somewhere).
Does anybody know where a second leisure battery would be in a Nuevo 2 if there was one?  Or has anybody seen a similar problem to this?

First you need to alter your profile, it shows you have a Talbot Rambler.

Your engine battery is under floor in front of passenger seat. Leisure battery under driver's seat and if you have a second leisure it may be under passenger seat or under a side bench (that should be easy to spot).

As Paul has said please up date your profile this will help us help you
it sounds like a basic re calibration on the controll

Hi folks, really sorry about the profile being out of date.  Swapped van a few years back, and in all the excitement, obviously never updated profile.   We have now found the driving battery under the floor (thanks, Paulmold!).   We then disconnected the leisure battery again, and found we could still turn lights on and still use water pump, so there must be more than one.   The leisure battery that we know about is under the side bench, but from what you've said, it sounds like we need to look under the driver's seat and we may find another one there!  Many thanks for your assistance, we will have a look this afternoon.

try the same test when its dark and no power from the solar or cover over the solar .are you using powering from leisure or vehicle battery

We have looked under the driver's seat, but there's only a tray containing jump leads and foot pump (assuming you don't mean taking the floor up, which looks like it would mean removing the seat first).   Under the passenger seat, there's a tool box.   under the floor near the passenger seat is the engine battery.   There's also another small floor panel near the passenger seat, but nothing obvious under it. 
I don't know if it makes any difference, but it is the layout with the living area at the back, so behind the driver's seat is the bathroom, then the wardrobe, and finally the living area.  The one leisure battery that we do know about is under the bench seat at the back.  
We have tried looking under the floor of the wardrobe, but could only see the water heater.
It's a good suggestion about trying it after dark.   The charger is (as far as we know) disconnected at the moment and we can still turn lights on, but trying it after dark will completely rule out the solar panel being involved.  
The switch for choosing between leisure battery and driving battery is on leisure battery.  We did try changing it just in case it is the wrong way round, but could still turn lights on with it in either setting.

The small panel in the floor is I believe to access the fuel sender in the fuel tank so don't remove that. Maybe that layout has it somewhere else if there is another battery .

If you do not have  manual  download  from the net

Thanks everybody for your suggestions. Testing has been a bit limited recently due to other things getting in the way, but we have found that with the leisure battery disconnected, and lots of lights turned on, after a few minutes, the lights went out, and the control panel changed to reading zero.  So, it would seem that either we have a large amount of capacitance in the system, or a very weak second battery.  

The suggestion of repeating the test after dark to eliminate the solar panel's input is a good one, and we will do that.

Glyne,  the suggestion of recalibrating sounds like it might be a help. Do you know if this is something we can do ourselves, or would we need to take it into a motor home servicing place?

Regarding checking the manual, always a good idea, but all it states is that if two batteries are fitted they must be the same type, doesnt mention their locations

I presume you know that there is a switch on the control panel that allows you to power all the habitation 12v electrics from the vehicle battery and that you are sure you have the leisure battery selected as the power source?

Thanks, yes, we have tried that in both positions just to be sure

If  you have  second leisure battery  surely  it would be at the end of  the  joining  cables from your  locker battery