Was scrubbing and steam cleaning the mould from the inside of my roof of my Westy the other day and noticed that the canvas is loose on one side. It seems to be fitted to something that should fit fixed to the roof, but the bit isn't.
There is a ally strip that screws to the both the fibre and tin part of the roof.
On the tin part the strip is on the inside and on the fibre its on the outside.
By the sounds of it you just need to screw it back in place.
IIRC where it connects to the fibreglass you should have an ally strip all the way along with screws holding it all together. Ally strip-canvas-fibreglass.....just as andy said above /\. Probably just a few of the screws have rusted away or come out over the past 20 years.....if you can't see it from inside your bus, look from outside......
Had a look today and it seems as though the canvas is wraped around a bar and stapled to it, as there are no screw or similar to hold it to the roof. There are some mesh "flaps" that seem to be attached the the bar and then are attached the roof. On of these has come loose.