These faults are a pain in the butt unless you have the right testing equipment.
Could be anything in the electrical circuit, feed from the battery or even the battery itself. Call on a mobile auto electrician they will find it. But first check all your fuses usually main fuse is the highest rated one.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix caravans/motorhomes.