I take it it leaks. If so, I would not use any tape, as it WILL come off in a few weeks.
Contact a specialist.
Try talking to "Loctite", the engineering glue people, they may have a product that works on "capillery action", that you could run along the top joint to fill any gap.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix caravans/motorhomes.