I have finally got round to sorting the camping electrics out so now I have internal lighting and a cooker along with eber

When the engine is running I am getting 13.8 volts at the leisure batt but with all my electrics running of the leisure batt the only thing it doesn't do is the engine,so I magine it will fail relatively quick enen though it's a 110amp/hr
So if anyone has fitted a batt charger permanently into there circuit what would you recommend?