When there is an annoying drip from the poptop roof to the main body of the caravan, try a peice of string to allow the water to run down onto the van.
If you have a roll out awning or an annex attached to the poptop roof you could consider using some dowling or similar to stop the roof from closing. Make sure you position this somewhere solid - not just on the aluminium cladding.
Some front caravan windows leak while travelling in wet weather, so try duct tape on the outside of the window frame, and/or a sheet of plastic inside (maybe taped to) the front window of the van.