Autosleeper Symbol leisure battery

I've had my van (2017 plate)  about 5 weeks now and been away a few times, on site.   I'd like to go off grid now but have no idea of the condition of my leisure battery or what size it is, and whether I should upgrade.  Most of you will think I'm an airhead as I have no idea where it is, I've looked, and online but no further forward.  Can you help out.   Where is it, should I upgrade?

Its usually under the settee on the left in a black plastic box

Thanks, will have a look tomorrow morning.

I'm very surprised that you did not get a user manual when you bought your Symbol.  Did you buy privately or through a dealer ?.  There is no reason that I can think of  why you should not have been given a manual on a 3 year old van.   Did you not get a comprehensive hand over when picking up the van?

page 9-28 of the manual says "under offisde sofa"

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