Please tell me I really need a battery charger!

I have just had a rather disappointing experience at a large Sheffield retailer. It was nearing closing time and I had had a very long journey to get there, thinking it would be the best place to buy a leisure battery. Nobody seemed to be able to properly advise me,and all just couldn't be bothered, but I went for the smaller of the two Lion batteries available. Hope that will be okay, but no idea. 

I then said that I understood that the batteries were powered from within the caravan but was told that this wasn't true and I would need a charger that I had to use to charge the battery at home.

Now, having done a bit of Googling, it seems this may not be correct. I didn't do too much Googling in advance, as I had been told this was THE place to go. ;-(

I am hoping someone can tell me that a) the small battery will be fine (6-berth van but no motor mover and not likely to be doing anything too wild and b) that I really need a battery charger because my car will be all the happier for it, as will the caravan.

{Clutching at straws...}

ps Do I need to check that it hasn't been allowed to run flat in the shop?