howcan i fix my A B I dalesman 420 2007?

Privae sale not much you can do but if you paid a lot for it then mention it to them.
If you paid a lot from a dealer get in touch mention the damp and say they should put it right free of charge. If they don't it's up to you which way you play it.

Meanwhile if you have to do this yourself re-seal using clear external sealant, all trims around the front and along the roof. in fact go around all at roof level. And get some heat in the caravan to dry out the patch.
Unfortunately many caravans suffer this after a while due to continual movement when towing. That's why the seal under the trims are like a none setting putty so it flexes in moving.
If you do it your self glass jar with water to wet finger/spatula. Rag and tissue. Tissue is to wipe excess off the mastic tube nozzle.

Don't cut the nozzle end to make opening bigger leave it as it is as its easier to control a thin layer than a big thick blob, it gets messy. Use wet finger or spatual to smooth out the mastic. Allow first coat to dry off dab with rag to clear any wet bits. then apply another layer. If you haven't used a mastic gun before make sure when completed each application you click off the small lock lever on the plunger rod at back of gun to take pressure off the mastic cannister, or it'll keep pushing out mastic.

Bob
March 2009