Low voltage

Replaced my old battery recently, fitted new battery and volt meter in the caravan shows 11.5 v, had none of these issues before battery was changed. Battery has full charge, van is hooked up to electric main as would be on site, and still only 11.5 v, volts drop when I switch lights on, so my thinking was that the on board charger may be failing?, but even so with ehu I should be showing more volts no?,what would be the best way to check this??