I had two roof leaks on my old SMB. Both were caused by workmanship issues.
The first was a pretty small leaks at first. And it is like any other water leak generally a pain in the ass. Where the water comes out many times is not where the water comes in. First I would check how level the top is where it was parked. If I was parked in my driveway (backed in) the leak would show towards the front of the van. Parked in front of the house the leak was in the back corner. It really depended on the angle the van was setting.
My Ford EB roof had ridges so this may not apply.
This was in the middle back of the van, the one bolt was for the J-Hook and the other to hold up the bed. At first glance it doesn't look like a problem . But if you look close, you can see the problem. The hole was not in the center, so the washer was not really laying flat in the trench.
The next one came much later, and was harder to find, I swore I kept getting water in through the the bolt holes. And by many indications that is where it would come out. But I found a small chip in the gel top, again didn't look like much but again parked in the driveway water just ran right to it.
I would say pull out the screws that are holding the roof panel and canvas up one at a time. Check the length, and put it back, and move on to the next. If you find one screw that is longer than the others make note to check out that location on the top. I had at least one longer screw used, same screw just a 1/4 in longer. Guess what, it caused a chip on the outside, and it was a pain to find.