it isn't the fact it drains quickly as much as leisure batteries are designed to go flat and recharge, normal ones give a short sharp release (to start) and then like to be recharged, letting normal batteries go flat to much is a recipe to kill them
I may be wrong (it has been known), but a 100Ah leisure battery and a 100Ah 'normal' battery will drain in the same amount of time if they have the same things running off then. I think the differences come with the longevity of the batteries and the charge cycles. I.e. LBs don't mind being drained completely and recharged while normal batteries aren't so keen on it. Both batteries will do the job but a LB will do it for longer.
i bought a new battery for the van so will be using the vans old main battery as my leisure battery for now. (until i buy a proper one) im only running the stereo and some led lights of it at the moment so will be interesting to see how long it will last when i start using it for charging and running other things!
im using a split charge relay so it should keep it topped up i hope but not used it properly yet to find out
I've bought 85Amp Hour ENGINE batteries for £25 from breakers and they've lasted for at least 4 years, including campsite use. In Halfords they sell for way over £100 I gave up with leisure batteries years ago as they never seemed to last long. The system works fine www.thelatebay.com/index.php?topic=17402
^^^ my thoughts too, if its surplus to requirements you can either sell it or use it, you might as well give it a go, you wont get your £80 back selling it.