Unfortunately, you signed a deal with incorrect info, so the dealer kindof has you at a disadvantage. If the ceiling appears to have a leak, then there is damage, no way around that. As long as the dealer is reasonable, you should continue with the deal you struck, otherwise you will be stuck with the total bill for the repair.
What this situation does highlight is just how important it is to check the roof often for any signs of cracks in the sealant. There are so many places that water can get in, and it is amazing how much water can get in even the tiniest of holes/cracks. And I speak from unfortunate experience, as we owned an older fifth wheel that had not been sealed properly right from the factory. It was a wood/tin camper, so luckily I could work on it, but ended up replacing the bottom half of the whole back end. It was actually kindof fun, a real learning experience, but fixing the ceiling is a whole different ball game.