Leaking roof product advice

We are currently doing a full refit of a coach built Mercedes 609d. After removing an inner fiberglass shell I’m finding some water is getting in along the roof seems. I have a mig welder but there’s no way I can reach the middle of the roof from above and the support beams block access from underneath. I’m looking for alternative ways of stopping the water.

I came across a product called Cromapol acrylic roof coating. Has anyone used it or is there a better alternative product?

Hi
I'm also new to the site and I'm surprised you haven't received a response yet.
I've come upon the product you mentioned and I think it spreads like paint.I opted for Tec 7 roof seal and is more like a paste that's applied with a stiff brush but I don't think it would spreead very well.Main components are bitumen and there's fibreglass also in it.Maybe that would suit for the inside seams.
I'm a newbie like yourself Hopefully there will be dome more advice coming shortly.

That is a structural failure, the roof needs to be welded again. You could fill it with sealant but I would think that over time the flexing that caused the initial cracking will cause more failure along the crack and leak again.
If you cannot reach the middle of the roof then you may need to make up some scaffolding or similar to allow access and welding.
If you cannot do it safely I would contract a decent welder that will be able to do the job, I don't think it will be that costly to do, and certainly cheaper than fitting an interior that needs to be stripped out when it leaks again.