I've got a small crack in my back panel to repair,I've bought some 2 part ABS bumper repair adhesive http://gt-pro.co.uk/powerfix-bumper-and-plastic-repair-cartridge.html to glue the crack together then fill and paint.It would help if you can access the crack from behind to reinforce,I can only drill an access hole through the back light housing so I am going to try to glue it first,I have put extra screw in already.Just witing for the weather to improve.
Ther is a good posting on another caravan forum detailing exactly how to do it that I am following