A tow bar fitter recently told me that if your leisure battery runs down or goes flat, then on start up, it will cause a power sure and blow a fuse. Anyway I got a couple of spares just in case.

Can't comment on the surge but to avoid damaging a leisure battery it shouldn't really be discharged any more than 50% and ideally given a regular 'proper' charge with something like a smart charger.
Correct me if I'm wrong but I think the inbuilt charger that operates when on hook up only provides a maintenance charge (something in the region of 13.7v) and not the full charge required for keeping the battery in top condition. A charging voltage of 14v+ does occur (on my van) but only when towing with the towing electrics connected.

As said above the trick is not to let it get into that position in the first place.