I have taken my van in for service and for some damp in the shower area to be checked. There is damp in the outer wall of the shower which is a separate standalone shower unit as we have a full width end bathroom.
The guy at the workshop has quoted me £1000 for the following:
Remove the pointless paper lining in the shower and clad the four walls in white plastic (will need the shower mount, door and tray removing to fit.
Remove half the awning rail (it's a twin axle and the rail is split in two meeting in the middle) reseal and refit - straightening in a few places.
Remove and reseal the ceiling vent in the shower.
Replace and reseal the external running light that attaches to the outer van wall.
i would be asking a few other places first, and get a better idea and what warranties for doing the job right first time they are going to give, unlike your £1000 guy who i doubt you will ever see again after hes got your money and the damp will be back within a couple of months time