If it leaks when raining have a look under the bonnet/hood at the windscreen/shield end around the body work and gutter channels the bonnet/hood fits into and around the wing area inside and see if what looks like drain slots are blocked. I had this prob on a Ford Sierra. Alternative the heater could be leaking.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix caravans/motorhomes.