I'm not very good with placing a link or whatever, but I'm sure someone else can, however failing that, have a quick search, it's been covered recently.
A deep-cycle gel battery is best for a leisure battery as it can be charged and discharged many times whereas a normal car battery can only be cycled a few times before it's knackered. Personally I just nick the Mini's battery when we go camping, that way I've got a spare if the main one packs up