what was the make of leisure battery you bought and what did you pay for it? if you dont mind me asking.
A split charging relay as fitted to most caravans and motor homes uses your cars/vehicles alternator to charge your leisure battery, your alternator puts out a charge of around 13.5v dc (the same as most battery chargers)
The leisure battery does not know what is charging it, the vehicle or a charger in your garage
as long as you charge your battery fully each time you will have no problems.
most standard battery chargers use smart charging technology nowadays so again you haVE no worries.
the scare mongering on various sites is in order to sell more batteries and chargers!!!!
hope this helps
regards Dave (qualified autoelectrician)