How do I get to the cross drilled ports to descale them the last owner of the caravan said the water was fine till about 2 months ago I believe the tank isn't filling as I have had the drain plug out is there any chance the old ball from the non return valve could still be in and stopping water from flowing as before I put the cold tap on without putting the drain plug in and it shot water across my garden but now I'm getting nothing when repeating last steps I'm new to this and totally confused I was going to buy a new waterheater but I would rather try repair it first as there a pretty penny