Similar problem with my 12yrs old Rapido!
Failed to solve the problem with silicon sealant, and then non setting mastic.
Due to slight movement in the joint, between front and back, the seals eventually broke down again.
I then tried a product called Eternabond on top of what was left of the mastic (after smoothing and tidying up the joint).
The technical spec. and endorsements are really impressive. Apparently used extensively in repair and maintenance of older RV's in America.
Not as neat a solution as a bead of sealant, but very effective and easy to apply, after thorough cleaning (with white spirit, or similar)! Recommended for joints where there is likely to be some movement.
So far so good, really pleased!!
Look here and scroll down to the 2nd item on the page!
I used EternaBond RoofSeal (White) Large Size 10cm x 7.6m £39.99