Keeping battery topped up during winter

Hi I just need to charge my caravans batt over winter in storage which also has alarm fitted, will 20 w be any good and does it req being outside or can i just put in front full sun facing window.

I was thinking about fitting the battery master to split the charge between vehicle and leisure battery.

on checking my Elddis the solar panel fitted has a Truma solar dual battery charger.

i have added a picture and looking at the cable connections it looks like it's just the leisure battery that's connected. So is it just a matter of connecting the vehicle battery to the dual charger so it charges both batteries?

Yes, connect a lead from the controller to the engine battery, select how much charge you want go to each battery. When one battery is full it should send full charge to the other and then you are done. 

I know it is remiss of me, but I should have said that my storage is inside a barn, so a large panel on the roof would be no good for me.

I do have a window behind the van facing south and I thought a panel could be fixed facing out and provide a decent charge.

There is a 10/12 watt panel available from Maplins and it seems to get good reviews, I will do some more reading make a decision.  Thanks for the replies. 

my Elddis Avanti 866 caravan battery is losing power even with a solar charger and connected to mains with only the frost protection on the intellgent controller being the only draw solar and mains charger are definetly working have tested with volt meter

if there is more going out than coming in (ie the battery is losing charge despite apparently charging) then perhaps the battery has a bigger drain than you think...?

turn off the battery charger and remove the fuse from the solar panel (ie remove all outside charging) and, after letting the battery rest for a while, test the drain with a multimeter that reads current....

you may find its far more than you think...eg the latest Pioneer DVD type head units can draw 2 amp in standby and 10 when in full chat....

If plugged into EHU it is hard to imagine charger can not manage any 12 volt demands

by recording the discharge and comparing it to the input charge level the OP wont have to 'imagine' anything....he'll have the answer.

this may of course be that the charger (or the SP, or even both) are not charging at the rate thought.

of course, the battery could be knackered and just isnt able to hold the supplied charge....

but, at the end of the day, if the battery isnt defective but is going flat, then the draw ('imagined' or otherwise) must be greater than the supplied charge....ots also possible that there is a 'short' somewhere which is causing the drain...

We haven't had the Motorhome on hook up since being on an Aire in Ypres back on the 31st of July - and that was the first time since April.

The Solar panel, two leisure batteries and starter battery, with help of the device I mentioned earlier on Page 1, mean I haven't needed hook up and rarely use EHU throughout the year. 

Modern vans, and not so modern vans, shouldn't need so much attention to the charging side when fitted with todays inexpensive technology.and a few hundred pounds, after the initial mortgage type purchase price of the van, spent on  ensuring you don't have to worry must be a good investment - isn't it?

by recording the discharge and comparing it to the input charge level the OP wont have to 'imagine' anything....he'll have the answer.

Do try and keep up BB! I was not responding to the OP but to Smowe!